Move the list of third-part plugins into the user manual

Problem

When searching for epub on https://rust-lang.github.io/mdBook/ there is no hit. When searching for pdf there is only one hit, but it does not show how.

Proposed Solution

IMHO it would be better to move the list of third-party-plugins from https://github.com/rust-lang/mdBook/wiki/Third-party-plugins to the user manual. That way it will be easier to find them.

Notes

No response