Update manual now stable can be installed with rustup

Update manual now stable can be installed with rustup

`rustup` can now install `rust-analyzer` for the stable tool-chain. This commit removes the note that `rustup` can only install for the nightly branch and adjusts the command.

I also added a note on how to find the path to the `rust-analyzer` binary when installed using `rustup`, and suggestions on how to work around it not being placed in `~/.cargo/bin`.

I thought it would be ideal to point everyone to use `rustup run stable rust-analyzer` to start `rust-analyzer`. That would make it trivial to switch to nightly however I could not get this to work in `nvim` therefore I left it as a suggestion at the end.