The "extra" generators were created in CMake's early years to provide
support for users of specific IDEs by directly generating their project
files alongside make or ninja files. Nowadays the file-api provides a
more generic, maintainable, well-tested, and robust way for IDEs to view
CMake project build trees. Deprecate the legacy "extra" generators to
encourage the corresponding IDEs to use the file-api.
Fixes: #19090
Improve formatting, primarily by:
* Adding links to relevant commands, properties, generators, and so on.
* Converting code, symbols, paths, and so on to fixed-width fonts.
* Hard wrapping lines to 80 characters or less.
Refactor edit_cache tool selection to ask each global generator for its
preference. Teach the Ninja generator to always use cmake-gui because
Ninja by design cannot run interactive terminal dialogs like ccmake.
Teach the Makefile generator to use cmake-gui when also using an "extra"
generator whose IDE has no terminal to run ccmake, and otherwise fall
back to CMAKE_EDIT_COMMAND selection for normal Makefile build systems.
