docs: explain conventional commits

Description

This MR clarifies the rules for choosing fix: and feat: prefixes in MRs/commits.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap

@aturinske @dmishunov @djadmin @elwyn-gitlab @ealcantara @erran @pslaughter @shekharpatnaik @tristan.read I'm pinging you as maintainers since I noticed that the last few version changelogs include many non-user-facing changes.