docs: clarify maintainers election process

Add a new page in TF-A documentation for clarifying the process to
elect a new maintainer. This builds on top of the Trusted Firmware
process [1], with the following TF-A specific details:

 - Must have contributed to the project for at least a couple of years.
 - Must dedicate at least 2 hours a week for maintainer duties.
 - Details about the election process. In particular, setting a
   one-calendar-week deadline for other maintainers to raise
   objections.

[1] https://developer.trustedfirmware.org/w/collaboration/project-maintenance-process/

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Change-Id: Ibef08bbbd4d18cd7aea13e01ba570972a7ee808d
3 files changed