Add instructions on how to deploy using github actions

[samuel-emrys]

There are currently instructions on how to deploy to GitHub Pages using Travis - it's probably more relevant to have instructions for GitHub Actions now that it's matured a bit. An exemplar implementation might be useful to get people up and running quickly

[bmcfee]

If it's at all helpful, I've just got this set up for one of my projects: https://github.com/librosa/librosa/pull/1447

It's probably too complicated to use as an example in the documentation, but it could be a good starting point if someone wants to simplify it for tutorial purposes. The main headache that I hit was getting the branches and tags to show up properly by using fetch-level and remote branches.

A few extra tweaks that I added to make things work how I wanted:

• The build pushes to a separate repository using the git-deploy action. This requires a deploy key and secret setup. A simpler version would just copy into the gh-pages branch.
• My build infers the latest tag (chronologically) and creates a symbolic link latest to the corresponding build. A top-level index.html then redirects to latest. This is brittle, and requires careful maintenance of the included versions to work properly, but it suits my needs.
• The triggers, right now, are on push and pull_request. This is temporary, and really just there to help development. I would recommend setting this to a release trigger instead.