[doc] Consistently use title and headings

* Add a "title" attribute to make the page title show up in the
  generated TOC. (The first heading isn't used there unfortunately.)
* The title serves as "h1" heading, use "h2" headings (`##`) and below
  within the document.

Signed-off-by: Philipp Wagner <phw@lowrisc.org>

doc/ug/vendor_hw.md

diff --git a/doc/ug/vendor_hw.md b/doc/ug/vendor_hw.md
index ba29437..8e33215 100644
--- a/doc/ug/vendor_hw.md
+++ b/doc/ug/vendor_hw.md
@@ -1,4 +1,6 @@
-# Work with hardware code in external repositories
+---
+title: Work with hardware code in external repositories
+---
 
 OpenTitan is not a closed ecosystem: we incorporate code from third parties, and we split out pieces of our code to reach a wider audience.
 In both cases, we need to import and use code from external repositories in our OpenTitan code base.