Promote reference docs pipeline out of pilot

[hey-august]

What this changes

Follow-up to #41, which added the API reference docs pipeline in its pilot configuration. This completes the promotion to production.

• Trigger builds on v* release tags (plus manual workflow_dispatch) instead of on the pilot branch, so the docs regenerate on a version bump rather than on every push. Thanks to @jpsantosbh for raising this during review.
• Replace mike-based versioning with a single unversioned site built via mkdocs gh-deploy.
• Add a favicon and address review nits: a more accurate docs_dir comment and removal of dead code in navbar.js.

Testing: validated end to end on my fork: dispatched the workflow from this branch and confirmed a green run (run link), a fresh gh-pages deploy, and the rebuilt site at hey-august.github.io/signalwire-python, including pages for modules added since the pilot (signalwire.ai_chat).

Checklist

No Python source or test changes in this PR; test items are N/A.

• bash scripts/run-ci.sh passes locally
• New tests assert on content (not just "does not raise" / "is not None")
• New test functions are type-annotated (mypy covers tests/)

Does this change public API?

• No
• Yes — naming it here so a maintainer can land the matching
  infrastructure change:

Changing something that goes on the wire?

If this alters an emitted shape, an enum value, or a parameter name, say where
you confirmed it — the generated types under signalwire/**/*_generated.py are
derived from the engine's schemas and are the authority here.