Improved crow/mustache.h file documentation

[8nvHHobXJ1KS]

I updated the documentation of crow/mustache.h and crow::mustache namespace. It now has many more entries in the reference:

• File crow/mustache.h
• Function crow::mustache::load
• Function crow::mustache::compile
• Function crow::mustache::default_loader
• Function crow::mustache::set_base
• Function crow::mustache::set_global_base
• Function crow::mustache::set_loader
• Function crow::mustache::load_text
• Function crow::mustache::load_text_unsafe
• Function crow::mustache::load_unsafe
• Enum crow::mustache::ActionType
• Class crow::mustache::Action
• Class crow::mustache::invalid_template_exception
• Class crow::mustache::rendered_template
• Class crow::mustache::template_t

Everything handwritten (as always).