Add a GitHub action for deploying releases to GitHub Pages

[mstenta]

Follow-up to #45

We currently publish a demo of the farmOS-map library on GitHub Pages at https://farmos.github.io/farmOS-map

This is still a manual process. It would be nice to automate that in the same way that we did for tagged release packaging in #45

[mstenta]

Bumping up this issue, since the demo at https://farmos.github.io/farmOS-map/ is now quite outdated. I used to update it manually before farmOS-map 2.x, and I haven't been doing it since.

Another consideration is we have multiple examples now (https://github.com/farmOS/farmOS-map/tree/2.x/examples) - do we want to publish them all separately? Or should we just publish one that has all features enabled like we used to? I would probably lean towards the latter as a first step myself.