ci: various improvements

[melekes]

1. add conventional-pr-title workflow
2. add markdown-linter workflow
3. cancel previous runs upon new commit
4. check deps weekly instead of daily
5. add mergify
6. add PR and issue templates
7. add "Fast CI" workflow (same as CI but only run when no changes to Dockerfile)

[andynog]

looks good to me, once merged we can run a manual workflow for testing

[melekes]

@sergio-mena can't merge here. For context: I've added test-fast, a faster version of test (I did not find a way to unite them). The correct thing is to require either of them to pass, not both.

[sergio-mena]

@melekes Fixed it by adding "test-fast" and removing "Test"