Use mdbook plugins in the OpenVMM Guide

[daprilik]

Some particularly interesting ones to look into:

• Link checking: https://github.com/Michael-F-Bryan/mdbook-linkcheck
  • Avoiding link-rot seems super useful
  • Would be cool to integrate with #149, in order to link to specific Rust types from the Guide
• Material Admonishments: https://github.com/tommilligan/mdbook-admonish
  • Useful for making warnings, notes, tips, etc... "pop" a bit better
• Mermaid Diagrams: https://github.com/badboy/mdbook-mermaid
  • Seems like a great tool for when we begin fleshing out our architecture docs
• Table of Contents Macro: https://github.com/badboy/mdbook-toc
  • Useful for chapter-specific intro pages