[doc] Fix stand-alone documentation build instructions
Signed-off-by: Colin O'Flynn <coflynn@newae.com>
diff --git a/doc/ug/documentation.md b/doc/ug/documentation.md
index 27d1f53..a1bd194 100644
--- a/doc/ug/documentation.md
+++ b/doc/ug/documentation.md
@@ -14,10 +14,12 @@
### Prerequisites
-Install the required tools by running the following command from the repository root.
+If you have already followed the [prepare the system]({{< relref "install_instructions#system-preparation" >}}) instructions you will have the required tools to build the documentation.
+
+If not, install the required tools by running the following command from the repository root:
```console
-$ sudo apt install curl python3 python3-pip
+$ sudo apt install curl python3 python3-pip doxygen xsltproc
$ pip3 install --user -r python-requirements.txt
```
