New documentation website is published!

[GeorgesStavracas]

Hello everyone,

I'm happy to say that the new documentation website is up and published: https://flatpak.github.io/xdg-desktop-portal/docs/

The documentation is now based on Sphinx, and that makes it much easier to add new pages and increment the documentation that is not generated by the XML files of D-Bus interfaces.

It should also make the D-Bus interfaces themselves a lot less convoluted, since they don't have to contain XML inside comment blocks.

Overall, I think this is a really nice addition. I'm sure there's a lot of follow-up work to be done here, but this transition should ease contributing to documentation enough for most people to contribute to it.

Thanks @ebassi for kickstarting this work!