Take version of `monad-bayes` package in the documentation as a variable

[mknorps]

    Can version of the documentation be taken as a variable? It would be a one less thing to remember when we bump the version.

Originally posted by @mknorps in https://github.com/tweag/monad-bayes/pull/179#discussion_r977437085

Also a check documentation if there other appearances of the version number.

[idontgetoutmuch]

I used to version the documentation for a big project with the git commit hash. Maybe we can add a tag (a good idea anyway) with the release version and then the docs can pick this up?