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 sassdoc 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://susy-docs.netlify.app/susy/docs
5. oddbird.net proxies the Netlify site under the /susy/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.

[jerivas]

@jgerigmeyer looks like I don't have push access to this repo. The oddleventy-branch will need to be created manually:

git checkout --orphan oddleventy-docs
git rm --cached -rf .
git commit --allow-empty -m "Initial commit"
git push -u origin oddleventy-docs

A new Netlify site will also need to be created based on that branch.