docs: minimize exposure of private links in public PRs and commits

Description

It is important that all information which is valuable for context about changes from PRs/commits should be publicly available in those PRs and commits for all community members to access and not be behind private links.

As per this change to OEP-51 and this discussion, the Open edX community is trying to ensure that private links and references are not included in the subject lines of the PRs and commits. Further such private references should only be present at the footers of the commit messages with them being tagged as such.

This MR adds/tweaks revelant sections in our handbook, so that we follow the recommendations of the community for our upstream PRs and commits as well.

Supporting information

Link to other information about the change, such as Jira issues, GitHub issues, or Discourse discussions. Be sure to check they are publicly readable, or if not, repeat the information here.

Testing instructions

Please provide detailed step-by-step instructions for testing this change.

Deadline

"None" if there's no rush, or provide a specific date or event (and reason) if there is one.

Other information

OpenCraft internal ticket BB-6536