`muse2 docs` command

[tsmbland]

It would be nice for users to be able to run a simple command (e.g. muse2 docs) to be able to open up the documentation. Ideally this would match the version of muse they're using, rather than just opening up the single version of the docs that we currently have online (see #1072)

There are a couple of ways that we could do this (see this comment for more details/challenges):

• Bundle the docs in the executable
• If we're doing Documentation page for each release #1072, then we can just open up the url for the relevant release