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:

• f0c54fb

[Veykril]

vscode and/or vscode insiders isn't strictly requirement (unless you want to work on the vscode extension)

[Veykril]

It would be fine to clarify that the setup is for vscode specifically though at the top (given the rest of this guide assumes it)

[davidkurilla]

I removed vscode and vscode insders as prerequisites. I added some information at the top stating that this workflow guide assumes that the user is using vscode and vscode insiders for this workflow.

[Veykril]

cargo xtask install --server --dev-rel

no need to build the extension unless you use it

[davidkurilla]

This workflow requires building a local installation of the extension. This would allow the user to view eprintln! debug statements in the Rust Analyzer Language Server output.