[Documentation:System] Improve Vagrant VM Install Docs (#591)

- Change steps under "Submitty Developer VM Installation" to be H4 to
create better separation between steps and sub-steps
- Add instructions for how to disable virtualization in Windows 11
- Consolidate steps for downloading and installing dependencies into a
single step to keep it short
- Fix various grammar and formatting mistakes

I had trouble following the installation steps as I was setting up the
VM. These changes make the steps easier to read and more digestible.

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Co-authored-by: Barb Cutler <[email protected]>
Co-authored-by: Barb Cutler <Barb Cutler>