[FEATURE] Deploy the latest stable version of the docs separately

[shonfeder]

As a followup to #400, we should also build and deploy and unstable version of the documentation.

(At some future point, we'll want to keep different versions of the manual available for different versions, but I think that's overkill before we are at major versions).

[konnov]

I thought that one has been solved already. If so, shall we close it?

[shonfeder]

The idea here was that we'd build and deploy two different versions of the docs: one for the latest stable release, and one that follows our trunk, and is updated continuously. Currently were only deploying the latter.

If we think we don't need to publish separate versions this way, and just continuously deploying from unstable is sufficient, then we can close this.

[thpani]

Since we're regularly cutting releases, in particular for breaking changes, and Apalache has reasonably matured, this does not seem to be an issue. Closing as wontfix for now.