Introduce feature version availability guidelines, add flags for 5.2

[Cheddam]

Description

Covers the changes suggested in #500, including tweaks to the contribution guidelines and PR checklist, and flags for most of the features introduced in 5.2.

Issues

• Include Available Version markers when documenting new features #500

Pull request checklist

• The target branch is correct
  • See branches and commit messages
• All commits are relevant to the purpose of the PR (e.g. no TODO comments, unrelated rewording/restructuring, or arbitrary changes)
  • Small amounts of additional changes are usually okay, but if it makes it hard to concentrate on the relevant changes, ask for the unrelated changes to be reverted, and submitted as a separate PR.
• The commit messages follow our commit message guidelines
• The PR follows our contribution guidelines
• The changes follow our writing style guide
• Code examples follow our coding conventions
• CI is green