Add docs previews

[ToucheSir]

Motivation and description

This would make (re)viewing docs changes much easier.

Possible Implementation

This has been done for a couple of the larger FluxML repos already. Copying and slightly modifying the configuration from those should be sufficient.

[Swish78]

Hi @ToucheSir ,

I'd love to contribute to it, but I'm not entirely clear on the specifics of the task. Could you please provide some more details on what needs to be done, especially regarding the configuration that needs to be copied and modified from the other FluxML repositories?

Any guidance or pointers to relevant documentation would be greatly appreciated!

[ToucheSir]

There are 3 main pieces:

1. Configure documenter and CI to push docs previews. You can see how that works current in Flux and consult the official docs at https://documenter.juliadocs.org/stable/lib/public/#Documenter.deploydocs.
2. Add a workflow to clean up docs previews. Again, see the existing Flux one and Documenter docs at https://documenter.juliadocs.org/stable/man/hosting/#Cleaning-up-gh-pages.
3. Add a PR comment workflow like https://github.com/FluxML/Flux.jl/blob/master/.github/workflows/pr_comment.yml.