Build out section for branch naming

What does this MR do?

As part of working Caps and special chars are OK but spaces are not (!118100 - merged) I realized the proposed one-line change to doc/user/project/merge_requests/creating_merge_requests.md pointed to a larger problem. Nowhere in https://docs.gitlab.com/ee/user/project/repository/branches/ had we laid out the full set of rules for branch names. The one line we were changing turned out to be incomplete, and the lack of a SSOT had caused multiple parts of the answer to be scattered across several pages in our documentation. I opted to merge the Community contribution merge request as-is, and focus on building a real solution here. This merge request:

• Revamps the Name your branch subheading of doc/user/project/repository/branches/index.md
  • Disallowed items: explain that branch names are subject to rules from Git and the rules GitLab layers atop the Git rules;
  • Optional but encouraged items: explain that carefully-structured branch names provide extra GitLab benefits, and link out to them
  • Acknowledge a 'safer' set of characters
  • Acknowledge that other packages may enforce additional rules, and link to a known set of rules for Docker
• Updates various pages that discuss branch naming to point back to this new SSOT

Related issues

• Related to Caps and special chars are OK but spaces are not (!118100 - merged) which this merge request is a follow-up to
• Closes Build better listing of branch naming expectations (#408882 - closed)

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