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

[bsipocz]

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.

[jsobeck]

Adding this comment about the change log at the suggestion of @eerovaher. It was not clear that a code contributor should modify the change log (concurrently) when submitting the commit (i.e., a reviewer/maintainer could be the only ones allowed to do this).

[bsipocz]

@jdavies-st - if you fancy writing infrastructure docs 😉