Add doxygen generation for vtkext module #5307

Job	Run time
Update LFS data cache	6s
webassembly	1m 34s
external-build	1m 27s
android (armeabi-v7a)	1m 42s
android (arm64-v8a)	1m 43s
android (x86)	1m 33s
android (x86_64)	1m 34s
windows (commit)	6m 47s
macos_arm (commit, no-bundle)	3m 40s
windows (v9.3.0)	6m 57s
macos_arm (v9.3.0, no-bundle)	3m 29s
windows (v9.2.6)	6m 52s
macos (commit, no-bundle)	9m 25s
macos_arm (v9.2.6, no-bundle)	3m 16s
linux (commit, standard)	4m 47s
macos (v9.3.0, no-bundle)	8m 20s
macos_arm (commit, bundle)	3m 59s
linux (v9.3.0, standard)	5m 1s
macos (v9.2.6, no-bundle)	11m 23s
linux (v9.2.6, standard)	4m 34s
macos (commit, bundle)	9m 20s
linux (v9.1.0, standard)	4m 5s
coverage	4m 52s
linux (v9.0.0, standard)	2m 50s
linux (headless, commit, raytracing, optional-deps, no-exclude-deprecated, egl)	4m 40s
python-packaging (ubuntu)	2m 59s
linux (exclude_deprecated, commit, raytracing, optional-deps, exclude-deprecated, no-egl)	4m 41s
python-packaging (windows)	4m 1s
sanitizer (address)	6m 27s
linux (no_optional_deps, commit, no-raytracing, no-optional-deps, no-exclude-deprecated, no-egl)	3m 6s
sanitizer (thread)	10m 24s
python-packaging (macos)	3m 34s
sanitizer (undefined)	5m 1s
static-analysis	3m 51s
	2h 38m 0s

Provide feedback