Use ReadTheDocs

Currently we're using Github Pages for hosting and Netlify for PR previews. We keep having issues due to Netlify's Python environment, and it's lack of support for Conda making our packages fall out of sync.

ReadTheDocs now supports Conda/Mamba, so we could simplify our environment, and have the same service manage the build and PR previews.

I remember we had some discussion about RTD before, and I can't remember exactly why we didn't use them and instead built our own versioning scheme. Was it due to the advertising? We can opt it out of advertising by someone getting a membership, which I just tried for Xpublish.

To get our existing old sites, we would probably want to tag the desired commits, then use post_build customization to pull those in. The other place to customize might be to use single version mode.

xref: #237 @emiliom @ocefpaf

Empieza revisando la configuración actual de GitHub Pages y Netlify; después, lee la discusión anterior en #237 y la documentación enlazada de ReadTheDocs sobre Conda, post_build y single-version mode. La tarea se consideraría completada cuando exista un plan de migración acordado que cubra las compilaciones, las vistas previas de PR y los sitios antiguos existentes; no se menciona ningún archivo de implementación ni ninguna prueba.