Include issue/PR templates

[jku]

tuf-on-ci-sign already uses .github/PULL_REQUEST_TEMPLATE/signing_event.md to fill in signing event contributions
There's a PR to handle .github/TUF_ON_CI_TEMPLATE/failure.md when filing new issues in update-issue action.

I suppose we should include those files in this repo? The files could be empty except for a <!-- comment --> explaining what they are