ci: better release workflow

[borisrizov-zf]

Summary

During the release of v0.3.0 (merge of develop into main) a couple of weaknesses have become apparent:

• The semantic release automation MUST be fed conventional commits, otherwise it won't build the correct application version or won't even trigger
• Mangled CHANGELOG.md when not following a procedure (this procedure was unclear up until now)

Solution

Add instructions in CONTRIBUTING.md:

• how to properly rebase branches before creating a PR
• pre-flight checks when creating a PR
• committer checklist before approving a PR

General next steps:

• REJECT PRs containing any kind of faulty git messaging
• The automatic commit on main must be immediately merged back into develop to avoid a messy CHANGELOG.md during the next release (potential to automate this)