docs: fix dead links in the PR template and backends docs (#11884)

- the PR template's 'Signed commits' anchor pointed at a CONTRIBUTING
  section that does not exist; repointed at the Commit messages
  section
- the longcat-video backend README linked a docs page that was never
  committed; replaced the dead link with plain text
- formal-verification/README.md used ../../../ for five in-repo
  packages (escaping the repo root); fixed to ../

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,7 +5,7 @@ This PR fixes #
 **Notes for Reviewers**
 
 
-**[Signed commits](../CONTRIBUTING.md#signing-off-on-commits-developer-certificate-of-origin)**
+**[Signed commits](../CONTRIBUTING.md#commit-messages)**
 - [ ] Yes, I signed my commits.
 - [ ] Documentation updated (docs/content/) for user-facing changes, or not applicable