cleanup doxygen and codespell

[gottfriedleibniz]

Fixes some missing/incorrect doxygen commands/labels. @mainpage in glm.hpp was also moved above @defgroup/@ingroup to avoid potential Doxygen member-group confusion with the last @defgroup command.

Few additional notes:

1. The doxyfile and doxygen.css could be updated to 1.9
2. Doxygen will spew typedef conflicts between gtc/matrix_integer.hpp and ext/matrix_*int*.hpp
3. There is some inconsistent description ordering in ext when compared to gtc and gtx, e.g., "Include <*.hpp> to use the features of this extension" versus "Defines functions that generate common transformation matrices"

[christophe-lunarg]

Thanks for contributing !