Add link from handbook to feature branch RP flow, post 18th

Why is this change being made?

Subject: Release Post flow

The current plan for 13.1 is to ask people who want to submit RP items, after the 18th at 8am PT, to create an MR targeted at the feature (release-X-Y) branch (instead of master).

This works with the following new section in our docs, https://docs.gitlab.com/ee/topics/git/feature_branch_development.html, which explicitly cites GitLab's release posts as the use case.

This would allow the Release Post Manager (and others) the opportunity to explicitly approve (or not) a new item, submitted after the noted time, for the upcoming release post.

I've also fixed an error with the TW Lead review of the MVP entry.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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