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/developer/index.rst b/docs/developer/index.rst
new file mode 100644
index 0000000..33a03cc
--- /dev/null
+++ b/docs/developer/index.rst
@@ -0,0 +1,26 @@
+Developer Docs
+==============
+
+.. toctree::
+    :maxdepth: 1
+    :caption: Contents:
+
+    arch-overview
+    project-structure
+    portability-model
+    service-access-protocols
+    service-locator
+    deployments
+    service-descriptions/index
+    software-requirements
+    build-instructions
+    running-tests
+    docs-build
+    writing-documentation
+    b-test
+
+--------------
+
+*Copyright (c) 2020-2021, Arm Limited and Contributors. All rights reserved.*
+
+SPDX-License-Identifier: BSD-3-Clause