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 ../

backend/python/longcat-video/README.md

@@ -11,7 +11,7 @@ RPC. It supports:
   systems such as NVIDIA DGX Spark.
 
 Install the `longcat-video` or `longcat-video-avatar-1.5` recipe from the
-LocalAI Model Gallery. See the [LongCat user guide](../../../docs/content/features/longcat-video.md)
+LocalAI Model Gallery. LongCat video backend
 for Studio and API examples, hardware requirements, and manual configuration.
 
 The upstream source is pinned in `Makefile` and patched at build time. The