Update documentation

[ehuss]

This is a large update to the documentation with the following changes:

• Move some of the duplicated information from the README, and consolidate the information in the guide.
• Rewrite the guide introduction. This includes a highlight of mdBook features.
• Add a User Guide introducing how to use mdBook.
• Add mdbook completions page.
• Consolidate the installation instructions in one place.
• Rewrite the Continuous Integration chapter, and move actual recipes to the wiki.
• Rewrite the preprocessor and backend documentation to avoid duplication and make them consistent. Each has the "configuration" page which describes how to configure them in book.toml, and a separate "for developers" page explaining how to create a plugin.
• General updates to configuration chapters.
• Update the markdown chapter with more information, and include extensions.
• More documentation on how code blocks work.
• Various other minor changes.

Closes #602
Closes #814
Closes #822
Closes #827
Closes #1159
Closes #1278
Closes #1008
Closes #1202
Closes #1367
Closes #1590

[joshrotenberg]

Nice!