Automatically publish oddleventy docs

[jerivas]

Description

Use GH Actions to automatically build and publish documentation on every release.

1. On each stable release the publish-docs.yml workflow is called
2. yarn herman outputs to the docs/ folder
3. The docs/ folder is pushed to the oddleventy-docs branch
4. Netlify picks up the new commit on this branch and re-deploys the site at https://blend-docs.netlify.app/blend/docs
5. oddbird.net proxies the Netlify site under the /blend/docs/ path (after we merge Use Netlify redirects to host docs oddleventy#268)

I also added the option of running the workflow on demand. This should allow us to rebuild docs without having to create a release.