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>