Related #11.
Builds docs to the gh-pages branch on every push to master. Also updates
all dead links to point to live ones (or ones that will be live after
this is merged).
Uses three third-party gh actions:
* leafo/gh-actions-lua@v8
* leafo/gh-actions-luarocks@v4
* peaceiris/actions-gh-pages@v3
* This receives secrets.GITHUB_TOKEN
Also switches to the new ldoc css and moves doc output folder to
./doc/out which is less confusing when configuring than ./doc/doc.
Once merged, docs will be available on https://excessive.github.io/cpml/
Currently, you can see the built docs on https://idbrii.github.io/love-cpml/
