Improve Qt documentation support

(+) Automatically find and import documentation if Qt distribution package is detected;
(+) Document a way to provide documentation data;
(+) Export description and detailed description for C++ enums and classes;
(+) Export description of enum variants.
(+) Don't show original C++ names of enum variants if none of the names were changed.
Make "C++ documentation" a link to the web site;
Replace <a> and <img> links with links to the web site;
If a C++ method or type is referenced in C++ documentation, append links to corresponding Rust methods after C++ documentation block.
Publish generated documentation and add a link to the readmes.

rust-qt / ritual