docs(versioning): add versioning to docs website

[niklas-wortmann]

No description provided.

[benlesh]

I'm not sure which one of these to look at? #5793

[niklas-wortmann]

Sorry @benlesh this one is the v7 docs merging in the master branch the other one is the V6 branch in the 6.x branch. The changes are pretty much identical though

[benlesh]

Overall, this looks fine to me... however, I do think we had discussed trying to do something with sub directories instead? I guess I would like to more about how all of this works. We're deploying more than one version of the site?

[benlesh]

I see.. it looks like angular is doing a thing with v6.angular.io, v7.angular.io, .... , angular.io (current), next.angular.io

I guess my only concern with that is it's not super intuitive to switch or know which one you're looking at. Particularly on mobile.

[benlesh]

This needs rebased as well. Overall LGTM.

[niklas-wortmann]

rebased. Just to sum up the plan I had for the docs versioning:
next.rxjs.dev always shows the docs of the upcoming major release
rxjs.dev shows the latest stable major version
vX.rxjs.dev will have the latest docs version for that major release

[benlesh]

Rebase and merge when you're ready, please.