Enforce PR merge standards via repo config + GitHub Issues

[sinopeus]

Currently we manually check that the following things are done before merging a PR:

1. Ensure there is a review and approval.
2. Use a squash commit.
3. Match the PR title and the squash commit message.
4. Follow the Conventional Commits standard for the squash commit message (and include the PR number at the end).
5. Delete the PR branch afterward.

Ideally this is automated instead of done manually.

[lsorber]

After investigation, it seems that:

1. (1), (2), and (5) could in theory be automated with GitHub Repository Settings, but it's probably better not to do this for a number of reasons (cf. feat: check if PR title conforms to Conventional Commits #312 for details).
2. (3) and (4) can be automated with a GitHub Actions Workflow (and implemented in feat: check if PR title conforms to Conventional Commits #312).

Unless you see alternatives for (1), (2), and (5), I would propose that we close this issue after #312.