chore: move docs from markdown files to rust source

[mladedav]

Closes #929

Motivation

Documentation created with include_str does not show in rust-analyzer tooltips. The issue is tracked by rust-analyzer but there's no indication anyone would be able to work on it anytime soon.

Solution

Move the documentation inline. handlers_intro.md and debugging_handler_type_errors.md were used in two places so there is some duplication but not too much, one is 7 and the other 40 lines including empty lines.

Interestingly, even clippy and rustdoc seems to not support include_str fully as they do not warn on included docs the same way as they do on inline docs.

[jplatte]

Thanks for taking care of this. I'll try to review in the next couple days.