docs: add instructions for building the docs

Although building these docs is mostly done through the standard
sphinx-build command, some additional dependencies and environment
variables are required for PlantUML and Graphviz generation to work
properly.

The first section is adapted from the main TF-M documentation.

Change-Id: I670b6361e9904166a3babccbabeac655f247699a
Signed-off-by: Nik Dewally <Nik.Dewally@arm.com>
diff --git a/docs/index.rst b/docs/index.rst
index dea768a..6c94274 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -1,6 +1,6 @@
-################################
+########################
 Trusted Firmware-M Tools
-################################
+########################
 
 .. toctree::
   :caption: Overview
@@ -19,6 +19,12 @@
   TF-M Manual Build <tf-m-manual-build>
 
 .. toctree::
+  :caption: Development
+  :titlesonly:
+
+  Building the Documentation <doc_build>
+
+.. toctree::
   :caption: Links
   :maxdepth: 1
   :hidden:
@@ -29,4 +35,4 @@
 
 --------------
 
-*Copyright (c) 2017-2023, Arm Limited. All rights reserved.*
+*Copyright (c) 2017-2024, Arm Limited. All rights reserved.*