doc: contribute: improve "Commit Guidelines" section

The current content does have all the critical information, but you
have to read carefully to catch several important details. This makes
it less likely that people will be able to follow the rules, which
wastes maintainers' time in PRs from new contributors, since we have
to explain the details repeatedly.

Fix it by making the introduction shorter while still covering all the
bases, and adding details with more concrete examples in the following
subsections.

Keep some other text in the document up to date and fix some
cross-references as needed.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>