Improve bootstrap instructions for MSYS2

* [x] Fix URLs on instructions page
* [x] Add links to serve configured pacman.d mirrorlist, with instructions on where to put them
* [x] Instructions on running MSYS2 through Command Prompt
* [x] Add `vim` to recommended `pacman` packages
* [x] Add instructions to upgrade to `msys2-runtime-3.6` instead
* [x] Instructions for setting up Rust environment