Write better PR templates

[jku]

• should make it clear that PRs for metadata changes should happen via signing events (so issues with a "sign/*" branch)

[jku]

My plan here is roughly:

• PRs from signers to a signing event: make the tool create the PR and use a custom template. This means root-signing-staging can include content in that template that e.g. pings the maintenance team to merge to the signing event theupdateframework/tuf-on-ci#209
• Generic PR template:
  • Include something like "If you are trying to contribute to an existing signing event, something has gone wrong"
  • As well as "If you are trying to start a signing event for an artifact change, please push a branch prefixed with sign/ to upstream instead"