Document the action workflow to deploy a specific commit for docs and napari

[willingc]

🧰 Task

See #release > 0.6.6 @ 💬

Follow up to #874.

---

Successfully deployed docs to website with the following inputs:

directory: 0.6.6
napari/napari: v0.6.6
docs: [40eb60428b094fee2950582b93eece466aa92e1c](https://github.com/napari/napari/commit/40eb60428b094fee2950582b93eece466aa92e1c) (commit before the version switcher PR)

• directory is the napari/napari.github.io repo's desired target directory.
• napari/napari is the desired ref which can be a tag or commit to use for napari
• docs: specific commit (or tag) to use with the docs repo build

[willingc]