doc: Move "About" content from index.rst to a new chapter

The index.rst page is now the primary landing page for the TF-A
documentation. It contains quite a lot of content these days,
including:

- The project purpose and general intro
- A list of functionality
- A list of planned functionality
- A list of supported platforms
- "Getting started" links to other documents
- Contact information for raising issues

This patch creates an "About" chapter in the table
of contents and moves some content there. In order,
the above listed content:

- Stayed where it is. This is the right place for it.
- Moved to About->Features
- Moved to About->Features (in subsection)
- Stayed where it is. Moved in a later patch.
- Was expanded in-place
- Moved to About->Contact

Change-Id: I254bb87560fd09140b9e485cf15246892aa45943
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/about/acknowledgements.rst b/docs/about/acknowledgements.rst
new file mode 100644
index 0000000..a9f6451
--- /dev/null
+++ b/docs/about/acknowledgements.rst
@@ -0,0 +1,21 @@
+Contributor Acknowledgements
+============================
+
+.. note::
+   This file is only relevant for legacy contributions, to acknowledge the
+   specific contributors referred to in "Arm Limited and Contributors" copyright
+   notices. As contributors are now encouraged to put their name or company name
+   directly into the copyright notices, this file is not relevant for new
+   contributions.
+
+- Linaro Limited
+- Marvell International Ltd.
+- NVIDIA Corporation
+- NXP Semiconductors
+- Socionext Inc.
+- STMicroelectronics
+- Xilinx, Inc.
+
+--------------
+
+*Copyright (c) 2019, Arm Limited. All rights reserved.*