docs: create setup documentation

[davidkurilla]

Created opinionated rust-analyzer workflow setup guide.

[rustbot]

There are merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

You can start a rebase with the following commands:

$ # rebase
$ git pull --rebase https://github.com/rust-lang/rust-analyzer.git master
$ git push --force-with-lease

The following commits are merge commits:

• f0c54fb6c0e6a59e2c0716fe9ad8f49ea6d53303