DOC: add brief devdocs to contributing.rst and main docs page

e.g.
- mention not using merge commits for updating branches

- mention that it's PR number that has to go into the changelog

- adding PR titles that appropriately describe the content (instead of referring to issue numbers). maybe even use prefixes for BUG, ENH, etc.

- no excessive style or infrastructure refactors please. automated PRs are not welcome even if your editor/IDE loves black/ruff/etc. Please set it up to honour our style check, and only style the lines you otherwise touch with your functional/enhancement PR.

Start by reading contributing.rst and the main documentation page to see their existing contributor guidance. Add the requested notes about avoiding merge commits, using the PR number in changelogs, descriptive PR titles, optional BUG/ENH prefixes, and avoiding unrelated style or infrastructure refactors. Done means all listed contribution practices are briefly documented in the requested locations.

Written by the indexing model from the issue text.