docs: Add development branch section

[jetm]

Description

development branch is an important part of the PR workflow, and it deserves its own section to explain its use in detail.

PR checklist

Please tick as appropriate and edit the reasons (e.g.: "backport: not needed because this is a new feature")

• [ ] changelog N/A
• [X] 3.6 backport done,
• [ ] 2.28 backport. Not required
• [ ] tests. Not required

Notes for the submitter

Please refer to the contributing guidelines, especially the checklist for PR contributors.

Help make review efficient:

• Multiple simple commits
  • please structure your PR into a series of small commits, each of which does one thing
• Avoid force-push
  • please do not force-push to update your PR - just add new commit(s)
• See our Guidelines for Contributors for more details about the review process.