docs(site): Add description of title-checker when submitting a PR (#3331)

site/content/docs/Contribution guidelines/_index.md

@@ -29,3 +29,9 @@ toc_hide: true
   - `make ci-tests`
   - `make lint-all` (if developing cross-platform code)
   - `make goreleaser`
+* When creating a PR, the title should match the pattern `<type>`(`<domain>`): `<description>` where:
+  - `<type>` is one of: `feat`, `fix`, `breaking`, `build`, `chore`, `docs`, `style`, `refactor`, `test`.
+  - `<domain>` is one of: `kopiaui`, `cli`, `ui`, `repository`, `snapshots`, `server`, `providers`, `deps`, `deps-dev`, `site`, `ci`, `infra`, `general`.
+  - `<description>` is a clear description of a PR.
+  - Follow the pattern precisely, as the title-checker cares about capitalization parentheses, and spaces.
+  - For example: `feat(cli): Add new policy rule --new-feature-x to enable using feature x`.