Restructure and redistribute std::path's module docs

Restructure and redistribute std::path's module docs

Part of #29368.
* Added a new summary paragraph about std::path's parsing facilities
* Slightly exanded `Component`'s docs
* removed the now redundant section on component types from the module docs
* moved the section on path normalization during parsing to the docs on
  `Path::components`
* Clarified difference between `Prefix` and `PrefixComponent` in their
  respecive summary sentences