Document the new library usage (import/require/global/... + new widget)

[raucao]

I think this should finally be done in the generated API docs, which are currently missing the whole intro for how to use the library.

[galfert]

Also document that users need to bring their own polyfill for promises now if they want to support older browsers.

[raucao]

I learned how Read the Docs works, and created a basic Sphinx setup with the RtD theme for our doc folder. I also converted the first two files to .rst in there, and thought about how to structure the docs. There's a special Contributing section now, so we can keep information for contributors seperate from library usage docs for app developers for example (and thus also create way more docs for contributing).

Currently pushing to a docs/sphinx branch.

[raucao]

I signed up for Read the Docs and imported the branch, so it should be built automatically when pushing to that now: https://remotestoragejs.readthedocs.io/en/docs-sphinx/

The "edit on GitHub" link is currently broken, because it has issues with slashes in branch names (they're converted to dashes over there).

[raucao]

Last update for today: I converted all existing files in doc to RST now (pandoc FTW!) and added all of them to the menu: https://remotestoragejs.readthedocs.io/en/docs-sphinx/

Automatic builds on push work like a charm, so anyone can now edit and create docs in that branch.

I think the next step would be adding a new "getting started" section and afterwards seeing how much of the existing content is still useful when editing it and what has to be deleted or rewritten entirely.