[docs] Document operational security policies for committers

Current documentation: https://docs.opentitan.org/doc/project/committers/

Introduce additional documentation covering the following topics:

* Committers audit policy. How often is the list reviewed, by whom.
* Security guidelines, policies and additional training required for committers.

This issue is under development and more details will be added over time.

Start by reading the current committer documentation at https://docs.opentitan.org/doc/project/committers/. Identify where the audit policy, security guidelines, and additional training requirements should be documented, then confirm the required review frequency, responsible people, and policy details. Done means the committer documentation covers all listed operational security topics.

Written by the indexing model from the issue text.