Better naming/categorization of API documentation

Summary
The titles of many API overview pages don't properly reflect what the pages actually contain. The pages should be renamed, or content should be separated out, or both.

Details

• The "<Qt Module> C++ Classes" page also contain Namespaces
  • It should be called "<Qt Module> C++ Classes and Namespaces"
  • Originally, the plan was to call it "<Qt Module> C++ API (comments, https://codereview.qt-project.org/#change,52610), but then it doesn't cover all C++ API
    • Enums, functions and macros are in the Namespace pages (e.g. http://qt-project.org/doc/qt-5.0/qtconcurrent/qtconcurrent.html)
  • e.g. http://doc-snapshot.qt-project.org/qt5-dev/qtmultimedia/qtmultimedia-module.html
  • https://doc-snapshots.qt.io/qt6-dev/classes.html Updated version
• The "All QML APIs by Module" page lists QML Types
  • https://doc-snapshots.qt.io/qt6-dev/modules-qml.html Updated version
  • Would the "QML Types" page ever grow to include anything other than QML types? If so, the lists should be named "<Qt Module> QML API". If not, the master list should be named "All QML Types by Module" for consistency
• QDoc is getting new variables which should be changed if the "C++ Classes" and "QML Types" pages are renamed
  • https://codereview.qt-project.org/#change,48379
  • navigation.cppclassespage
  • navigation.qmltypespage
• The QtGlobal Header page is like a Namespace page
  • http://qt-project.org/doc/qt-5.0/qtcore/qtglobal.html
  • (Probably not an issue)
• The "Qt Reference Pages" page lumps Modules, Namespaces, and QML API with C++ Classes
  • https://doc-snapshots.qt.io/qt6-dev/reference-overview.html Updated version
• The "Class and Function Documentation" page lumps Modules, Namespaces, and QML API with the C++ Classes
  • http://qt-project.org/doc/qt-5.0/qtdoc/classlists.html
  • Furthermore, its URL is "classlists.html"
  • https://doc-snapshots.qt.io/qt6-dev/reference-overview.html#class-lists Updated version 
• The "All Functions" page lumps Macros with Functions
  • http://qt-project.org/doc/qt-5.0/qtdoc/functions.html
  • https://doc-snapshots.qt.io/qt6-dev/functions.html Updated version
  • They should be separated, or renamed "All Functions and Macros"
• The "All Classes by Module" page includes...
  • The "<Qt Module> C++ Classes" pages (which actually includes Namespaces too)
  • The "Class and Function Documentation" page (which includes Functions, and isn't grouped by Module)
  • https://doc-snapshots.qt.io/qt6-dev/whatsnew60.html#new-classes-and-functions Updated version

Appendix
The current (partial) doc hierarchy is:

- MODULE LANDING PAGE
    - C++ CLASSES
        - C++ class list
        - C++ namespace list
            - C++ type (enum) list
            - C++ function list
            - C++ macro list
    - QML TYPES
        - QML type list