Add hosted API documentation

[dstansby]

The documentation is currently set up in the docs directory, but isn't automatically built or published. This can't be done with readthedocs becuase it's not possible to install xvfb there. Instead the docs should be built on GitHub actions and published ot a GitHub pages repository.

[dstansby]

I think it's now possible to use xvfb on readthedocs, so it would be best to use readthedocs instead of github actions.

[dstansby]

I'm having issues with readthedocs, specifically getting it to work without x-server erroring after the build. See #83 for my attempt.

I think it's better to pivot back to using GitHub pages for this, especially since that's what the official napari docs use, so we have an example to follow there and more likeley to have debugging buddies if we do run into any issues.

[samcunliffe]

My most recent example of doing this is in WAZP🐝.

[dstansby]

Nice! Maybe a good topic for our in-person dev morning.

[dstansby]

I've pivoted back - since the example gallery build doesn't work in GH action either, I figure it's best to disable the gallery build and just use readthedocs for now.