DOC: update documentation.

Add documentation covering:
  - developer information about architecture
  - project structure and building
  - project overview and service descriptions
  - the portability model for supporting hardware

Signed-off-by: Julian Hall <julian.hall@arm.com>
Co-Authored-By: Gyorgy Szing <gyorgy.szing@gmail.com>
Change-Id: I8bf9c01a66350719d82a7ca2bc1c78a8ab17978d
diff --git a/docs/index.rst b/docs/index.rst
index 9a643c4..cb44e2c 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -4,16 +4,67 @@
 root-of-trust services across a range of secure processing environments such as those
 provided by OP-TEE and Hafnium.
 
+.. raw:: html
+
+    <ul class="grid">
+        <li class="grid-item">
+            <a href="overview/index.html">
+                <img alt="" src="_static/images/tfm.png"/>
+                <h2>About trusted services</h2>
+            </a>
+            <p></p>
+        </li>
+        <li class="grid-item">
+            <a href="project/index.html">
+               <img alt="" src="_static/images/tfm.png"/>
+               <h2>About the project</h2>
+            </a>
+            <p></p>
+        </li>
+        <li class="grid-item">
+            <a href="developer/index.html">
+                <img alt="" src="_static/images/tfm.png"/>
+                <h2>Developer Docs</h2>
+            </a>
+            <p></p>
+        </li>
+        <li class="grid-item">
+            <a href="environments/index.html">
+                <img alt="" src="_static/images/tfm.png"/>
+                <h2>Processing Environments</h2>
+            </a>
+            <p></p>
+        </li>
+        <li class="grid-item">
+            <a href="security/index.html">
+                <img alt="" src="_static/images/tfm.png"/>
+                <h2>Security Model</h2>
+            </a>
+            <p></p>
+        </li>
+        <li class="grid-item">
+            <a href="standards/index.html">
+                <img alt="" src="_static/images/tfm.png"/>
+                <h2>Standards Compliance</h2>
+            </a>
+            <p></p>
+        </li>
+    </ul>
+
 .. toctree::
    :maxdepth: 1
    :caption: Contents:
    :hidden:
 
-   project/license
-   project/maintainers
+   overview/index
+   project/index
+   developer/index
+   environments/index
+   security/index
+   standards/index
 
 --------------
 
-*Copyright (c) 2020, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2020-2021, Arm Limited and Contributors. All rights reserved.*
 
 SPDX-License-Identifier: BSD-3-Clause