docs(HACKING): update commit message guidelines

[mr-cal]

• [x] Have you followed the guidelines for contributing?
• [x] Have you signed the CLA?
• [x] Have you successfully run tox?

---

Updates to commit message guidelines to be more descriptive and authoritative.

Preview RST here

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (0d328c4) 100.00% compared to head (58777ac) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #162 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 1 1 Lines 10 10 Branches 2 2 ========================================= Hits 10 10 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[mr-cal]

Should we add a merge type for PRs like this? https://github.com/canonical/craft-parts/pull/612

[syu-w]

That maybe chore? Since it self not include anything.