[doc] Prefer relative links in Markdown

Relative links make it easier to relocate files, prefer using them where
possible.

Signed-off-by: Philipp Wagner <phw@lowrisc.org>
diff --git a/hw/ip/otbn/doc/_index.md b/hw/ip/otbn/doc/_index.md
index 5a1085c..6a8e37a 100644
--- a/hw/ip/otbn/doc/_index.md
+++ b/hw/ip/otbn/doc/_index.md
@@ -48,7 +48,7 @@
 # Instruction Set
 
 OTBN is a processor with a custom instruction set.
-The full ISA description can be found in our [ISA manual]({{< relref "hw/ip/otbn/doc/isa" >}}).
+The full ISA description can be found in our [ISA manual]({{< relref "isa" >}}).
 The instruction set is split into two groups:
 
 * The **base instruction subset** operates on the 32b General Purpose Registers (GPRs).
@@ -94,7 +94,7 @@
 
 OTBN has an in-built call stack which is accessed through the `x1` GPR.
 This is intended to be used as a return address stack, containing return addresses for the current stack of function calls.
-See the documentation for [`JAL`]({{< relref "hw/ip/otbn/doc/isa#jal" >}}) and [`JALR`]({{< relref "hw/ip/otbn/doc/isa#jalr" >}}) for a description of how to use it for this purpose.
+See the documentation for [`JAL`]({{< relref "isa#jal" >}}) and [`JALR`]({{< relref "isa#jalr" >}}) for a description of how to use it for this purpose.
 
 The call stack has a maximum depth of 8 elements.
 Each instruction that reads from `x1` pops a single element from the stack.
@@ -291,7 +291,7 @@
 
 OTBN has 256b Wide Special purpose Registers (WSRs).
 These are analogous to the 32b CSRs, but are used by big number instructions.
-They can be accessed with the [`BN.WSRRS`]({{< relref "hw/ip/otbn/doc/isa#bnwsrrs" >}}) and [`BN.WSRRW`]({{< relref "hw/ip/otbn/doc/isa#bnwsrrw" >}}) instructions.
+They can be accessed with the [`BN.WSRRS`]({{< relref "isa#bnwsrrs" >}}) and [`BN.WSRRW`]({{< relref "isa#bnwsrrw" >}}) instructions.
 
 <table>
   <thead>
@@ -309,7 +309,7 @@
       <td>RW</td>
 <td>
 
-The modulus used by the [`BN.ADDM`]({{< relref "hw/ip/otbn/doc/isa#bnaddm" >}}) and [`BN.SUBM`]({{< relref "hw/ip/otbn/doc/isa#bnsubm" >}}) instructions.
+The modulus used by the [`BN.ADDM`]({{< relref "isa#bnaddm" >}}) and [`BN.SUBM`]({{< relref "isa#bnsubm" >}}) instructions.
 This WSR is also visible as CSRs `MOD0` through to `MOD7`.
 
 </td>
@@ -356,7 +356,7 @@
 
 ### Loop Stack
 
-OTBN has two instructions for hardware-assisted loops: [`LOOP`]({{< relref "hw/ip/otbn/doc/isa#loop" >}}) and [`LOOPI`]({{< relref "hw/ip/otbn/doc/isa#loopi" >}}).
+OTBN has two instructions for hardware-assisted loops: [`LOOP`]({{< relref "isa#loop" >}}) and [`LOOPI`]({{< relref "isa#loopi" >}}).
 Both use the same state for tracking control flow.
 This is a stack of tuples containing a loop count, start address and end address.
 The stack has a maximum depth of eight and the top of the stack is the current loop.
@@ -468,7 +468,7 @@
 
 Whenever OTBN observes an error, it will generate an alert.
 This gets sent to the alert manager.
-The alert will either be fatal or recoverable, depending on the class of error: see [Alerts]({{< relref "hw/ip/otbn/doc#alerts" >}}) and {{< regref "ERR_BITS" >}} below for details.
+The alert will either be fatal or recoverable, depending on the class of error: see [Alerts]({{< relref "#alerts" >}}) and {{< regref "ERR_BITS" >}} below for details.
 
 If OTBN was running when the alert occurred (this is true whenever {{< regref "STATUS.busy" >}} is high), it will also:
 - Immediately stop fetching and executing instructions.
@@ -480,7 +480,7 @@
 In this case, the {{< regref "ERR_BITS" >}} register will not change.
 This avoids race conditions with the host processor's error handling software.
 However, every error that OTBN detects when it isn't running causes a fatal alert.
-This means that the cause will be reflected in {{< regref "FATAL_ALERT_CAUSE" >}}, as described below in [Alerts]({{< relref "hw/ip/otbn/doc#alerts" >}}).
+This means that the cause will be reflected in {{< regref "FATAL_ALERT_CAUSE" >}}, as described below in [Alerts]({{< relref "#alerts" >}}).
 This way, no alert is generated without setting an error code somewhere.
 
 <div class="bd-callout bd-callout-warning">
@@ -556,7 +556,7 @@
 # Writing OTBN applications {#writing-otbn-applications}
 
 OTBN applications are (small) pieces of software written in OTBN assembly.
-The full instruction set is described in the [ISA manual]({{< relref "hw/ip/otbn/doc/isa" >}}), and example software is available in the `sw/otbn` directory of the OpenTitan source tree.
+The full instruction set is described in the [ISA manual]({{< relref "isa" >}}), and example software is available in the `sw/otbn` directory of the OpenTitan source tree.
 
 A hands-on user guide to develop OTBN software can be found in the section [Writing and building software for OTBN]({{<relref "doc/ug/otbn_sw.md" >}}).
 
@@ -580,7 +580,7 @@
 
 ## Returning from an application
 
-The software running on OTBN signals completion by executing the [`ECALL`]({{< relref "hw/ip/otbn/doc/isa#ecall" >}}) instruction.
+The software running on OTBN signals completion by executing the [`ECALL`]({{< relref "isa#ecall" >}}) instruction.
 
 When it executes this instruction, OTBN:
 - Stops fetching and executing instructions.
diff --git a/hw/ip/otbn/doc/dv/index.md b/hw/ip/otbn/doc/dv/index.md
index 2e15455..d1b7159 100644
--- a/hw/ip/otbn/doc/dv/index.md
+++ b/hw/ip/otbn/doc/dv/index.md
@@ -18,7 +18,7 @@
 ## Design features
 
 OTBN, the OpenTitan Big Number accelerator, is a cryptographic accelerator.
-For detailed information on OTBN design features, see the [OTBN HWIP technical specification]({{< relref "hw/ip/otbn/doc" >}}).
+For detailed information on OTBN design features, see the [OTBN HWIP technical specification]({{< relref ".." >}}).
 
 ## Testbench architecture
 
diff --git a/hw/ip/otbn/doc/isa.md b/hw/ip/otbn/doc/isa.md
index 9d18d1d..0c267e2 100644
--- a/hw/ip/otbn/doc/isa.md
+++ b/hw/ip/otbn/doc/isa.md
@@ -3,7 +3,7 @@
 ---
 
 This document describes the instruction set for OTBN.
-For more details about the processor itself, see the [OTBN Technical Specification]({{< relref "hw/ip/otbn/doc" >}}).
+For more details about the processor itself, see the [OTBN Technical Specification]({{< relref "." >}}).
 In particular, this document assumes knowledge of the *Processor State* section from that guide.
 
 The instruction set is split into *base* and *big number* subsets.