ci: semantic PR title via conventional commits

[scarf005]

Purpose of change

• enforce conventional commits for PR titles
• remove redundant ## Summary

Describe the solution

install https://github.com/apps/semantic-prs to enforce PR title

Describe alternatives you've considered

pain

Testing

used them on other projects, they work as intended

Additional context

Q: Nooo, do i have to edit all those PR titles, i don't know how to do that
A: No worries, i've been doing that manually for months and this app just prevents me from overlooking one

Q: Then can i just write whatever on PR title and you'll fix it for me?
A: It'd be very nice if you could follow conventional commits, tho.

Checklist

• reword automated i18n PR title
• remove ## Summary section
• think up of better category
• update docs

Additional Context

Semantic PR bot also needs to be approved to enforce the PR title consistency.

[Coolthulhu]

I'm not sure if I understand this bit.
Also, seems to be a typo: unrelevAnt.

[scarf005]

oh, i meant pruning html comments in PR body, as they get included in PR body previews (e.g on notification)

[olanti-p]

Before #3519, the template used to ask to remove the comments, but turns out ~40% of PRs don't do that anyway (either laziness or just avoiding removing something that may become relevant later), so we're stuck with them until GitHub introduces issue forms, but for PRs.

[Coolthulhu]

Looks good to me, I'll approve the semantic PR thing for now. But I'd like to know what exactly is the "no comment" section about - I hope it doesn't mean that the submitter will have to manually prune the PR notes.