Migrate to VitePress, include rs.js docs

[raucao]

See https://community.remotestorage.io/t/new-website-and-rs-js-documentation-site/850 for details and https://rs-website-preview.5apps.com/ for current live preview.

[raucao]

I think we're done for this PR, not withstanding further improvements afterwards.

See the updated README for instructions for local previews.

I added a GitHub action, which should deploy the new website to GitHub pages once this PR is merged, and then whenever the master branch is updated.

[DougReeder]

In order to run git submodule update --init, you must have have a public SSH key associated with your GitHub account. That's a reasonable requirement, but it needs to be documented.

[raucao]

@DougReeder Resolved in the latest commit. Thanks.

[DougReeder]

It looks good. As soon as the instructions at rs.js/docs/contributing/docs.html are replaced, we're good to go.

[raucao]

@DougReeder OK, thanks. Opened remotestorage/remotestorage.js#1317 for it.

I edited it directly on GitHub, just to test that it's possible/easy to contribute docs changes without even setting up the local preview.

[raucao]

I fixed a few minor issues directly on master after merging this PR. We're live!