Docs: Add guidance for CHANGELOG entries

**What is the current behaviour and why should it be changed?**

We currently don't have clear guidance when to add changelog entries.

**Describe possible approaches**

- [ ] What's the goal for the CHANGELOG, who is the audience?
- [ ] Should internal changes (i.e. those which do not directly affect generated artifacts) be documented in the CHANGELOG? Options:
1. Document all PRs.
2. Document PRs which *may* be user-visible (i.e. including third-party library bumps, build logic changes)
3. Document PRs which intended user-visible changes only (bug fixes, new features, performance changes).
- [ ] How should entries be written? (past tense)
- [ ] How are related PRs handled?
- [ ] Where should this guidance be stored? PR template?

**Has this feature been discussed and generally agreed?**

One of the previous discussions around this topic: https://github.com/jamulussoftware/jamulus/pull/2778#issuecomment-1213473371

Start by reading CONTRIBUTING.md and the discussion linked from PR #2778. Resolve the open questions about the CHANGELOG audience, which changes require entries, entry style, related PRs, and where the guidance belongs. Done means the project has agreed, actionable guidance in the selected location.

Written by the indexing model from the issue text.