Redo of unversioned workflow

[melissawm]

Hi all - this is a new version of the unversioned workflow.

I have confirmed with a deployment to my fork: https://melissawm.github.io/napari.github.io/stable/index.html

This PR:

• Removes stable developer pages before copying latest: pages that were moved or deleted were lingering in the stable folder.
• Fix commit command: allow for empty commits (for example, if we manually want to re-reploy pages) and include newly created files in the commit (we were not doing that previously and were missing the files that were recently moved to subfolder under the developer folder)
• Use sed to replace css in all unversioned pages (use dev version of css for dev versions of pages)
• Include naps, release notes and roadmaps in unversioned pages (prevents breakage since this files are all in the developer ToC.

We may want to move naps, release notes and roadmaps to be under the developer folder in the future, but this action will work with the current folder structure.

[melissawm]

I am self-merging with two approvals and the discussion in the docs working group meeting. I'll keep an eye out for potential issues and can revert if that's the case.

[melissawm]

It looks like we will need a

on:
  deployment

trigger in addition to the on: push. See https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#deployment

I'll send a PR as soon as I confirm the current action is working correctly, as I deployed it manually to test.