Condense and remove redundancy in CONTRIBUTING.md

Now that we've merged HACKING.md with CONTRIBUING.md (https://github.com/canonical/operator/pull/2734), there's some duplicative content, and I believe it could also go on a diet (trim some of the overly verbose content, remove "obvious" things, and so on).

Read CONTRIBUTING.md and review the HACKING.md merge in pull request #2734 to identify duplicated material. Trim verbose or obvious guidance while preserving the useful contribution instructions; done means the document is shorter, non-redundant, and still covers the needed contributor information.

Written by the indexing model from the issue text.