chore: add checks + conventional commit comment to PR template

[awphi]

Changes

• Adds a check list of items to the PR template to remind us to test and update docs (mostly focused on the latter w/ the introduction of docs: structure docs for public docs site, add basic introduction section #336)
• Adds a comment to remind contributors to use conventional commits as the PR title

This makes our PR template identical with the other OSS product we maintain; remoteproc-runtime.

Checklist

• 🤖 This change is covered by tests as required.
• 🤹 All required manual testing has been performed.
• 📖 All documentation updates are complete.

[yejseo01]

I guess this will remind people to update docs

[awphi]

I'm not a huge fan of this either. Let's give it a go and if it proves ineffective at reminding us then we can think about more involved means.