Add build GitHub action

[ashawley]

This is just based on the Ruby action:

https://github.com/actions/starter-workflows/blob/d037e97/ci/ruby.yml

[ashawley]

This is the first installment of trying to solve #388.

[ashawley]

I suppose we could use the Jekyll action, but I'm pretty sure we need to have more control over the build than that going forward. Maybe the Jekyll action could be used to deploy the gh-pages.

[ashawley]

It ran for this pull request:

https://github.com/johno/pixyll/pull/419/checks?check_run_id=511235667

[ashawley]

I'll follow-up, and see if caching may be able to make this run slightly faster:

https://github.com/actions/cache/blob/master/examples.md#ruby---bundler

[kosperera]

@ashawley I tried a combination of what you did for the build.yml script and another GitHub Action peaceiris/actions-gh-pages@v3. It seemed to do the job, at least for this repo:
https://github.com/kosalanuwan/bookworm/blob/master/.github/workflows/publish.yml

Notes:

• The repo is a detached fork of Pixyll v2 and v3.
• gh-pages only contains the _site binaries and not a copy of the master