Add mdbook-toc

[igaray]

mdbook-toc is an mdbook preprocessor that allows auto-generating TOCs. This is useful for long sections with many subtitles.

• Add the preprocessor configuration to book.toml
• Add the preprocessor to the CI configuration
• Add the preprocessor dependency and usage to the instructions on how to contribute to the guide.
• Identify sections in need of a TOC and add the <!-- toc --> marker

[jyn514]

Related: #853