Add GitHub Actions workflow to build and publish docs to GitHub Pages

[esabol]

This PR adds a GitHub Actions workflow to build and publish docs to GitHub Pages.

No idea if this will work. GitHub Actions are difficult to test before merging. I did test building the docs in a local Docker container, but there are many differences with the GitHub Actions environment.

[SpamapS]

I'm a little confused.. where does this publish to?

[esabol]

I'm a little confused.. where does this publish to?

The GitHub Pages URL for a given repository is always https://{userid}.github.io/{reponame}, so it should publish to https://gearman.github.io/gearmand.

[SpamapS]

Cool, well let's see what it does!

[SpamapS]

https://gearman.org/gearmand/ has the docs! I had to bump the version of upload-pages-artifact which was confusingly annoying (see latest commit in master). Anyway, thanks! If you want to add links to it to the website you should have access to that repo as well. https://github.com/gearman/gearman.github.io just make the links relative.. to /gearmand.

[esabol]

I'm surprised I got so close to getting it fully working before it was merged!

Need to upgrade to actions/configure-pages@v5 to eliminate the Node16 deprecation warning. And the environment needs a tweak, I think. I'll submit another PR soon.