chore: Update PR template for reviewer focus

[distantnative]

Review

• Design & concept: Judge the approach, the API and the naming. Implementation details matter less.

Timing: whenever

Description

• AReview section at the top where the creator says what he needs from the reviewer(s): a quick look, an opinion on the concept, or actually following the logic line by line.
• Changelog: the nine section headings moved into the comment as a list to copy from, instead of being pre-printed.
• Changelog: added 🔒 Security and 🏎️ Performance
• Benchmarks: a subsection under Description for PRs that claim a performance gain
• Docs: for new features, also explain why a user would want this and show one real-world example.

Targeting the main branch as this doesn't need to be part of a release and allows us to get using the new template as soon as possible on all PRs.

[bastianallgeier]

I think I would already ditch the descriptions. We get so few external reviews, that this is really more for us.