doc: Refactor contributor acknowledgements

- Make the list of contributors into an actual list
- Use note syntax for the note
- Remove the Individuals heading since there are none

This file could be considered for removal as it is a legacy
document, as its note explains.

Change-Id: Idf984bc192af7a0ec367a6642ab99ccccf5df1a8
Signed-off-by: Paul Beesley <paul.beesley@arm.com>

docs/acknowledgements.rst

 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.**
-
-Companies
----------
-
-Linaro Limited
-
-NVIDIA Corporation
-
-Socionext Inc.
-
-Xilinx, Inc.
-
-NXP Semiconductors
-
-Marvell International Ltd.
-
-STMicroelectronics
-
-Individuals
------------
+.. 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.