add documentation for each module

[tg]

We should somehow document each module so users know what they're for and why they're important. I had a will of describing new modules during the release notes, but these messages are not consumed by many people and are not long lasting – instead with every new module we should link to a documentation.

This could be either a single .md file, a series of files for each mode, or a wiki. I guess ideally these should be also available directly via CLI.