Add Doxygen build instructions to docs/README.md #2533

Job	Run time
clang-format	12s
python-lint	6s
ubuntu-clang-build (ubuntu-24.04-arm)	2m 36s
ubuntu-gcc-build (ubuntu-24.04-arm)	2m 58s
ubuntu-clang-build (ubuntu-24.04)	2m 9s
windows-clang-build	4m 28s
ubuntu-gcc-build (ubuntu-24.04)	1m 46s
macos-clang-build	2m 8s
windows-msvc-build	3m 6s
macos-clang-test	3m 0s
ubuntu-gcc-test (ubuntu-24.04)	4m 27s
ubuntu-gcc-test (ubuntu-24.04-arm)	5m 29s
ubuntu-clang-test (ubuntu-24.04-arm)	5m 52s
ubuntu-clang-sanitizer-build (ubuntu-24.04)	1m 17s
ubuntu-clang-test (ubuntu-24.04)	4m 43s
ubuntu-clang-sanitizer-test (ubuntu-24.04)	2m 2s
windows-msvc-test	2m 16s
macos-clang-test-extended	2m 13s
windows-clang-test	1m 17s
windows-msvc-test-extended	1m 35s
ubuntu-gcc-test-extended (ubuntu-24.04-arm)	2m 58s
ubuntu-gcc-test-extended (ubuntu-24.04)	2m 37s
ubuntu-clang-test-extended (ubuntu-24.04-arm)	3m 55s
ubuntu-clang-test-extended (ubuntu-24.04)	2m 46s
ubuntu-clang-sanitizer-test-extended (ubuntu-24.04)	1m 50s
windows-clang-test-extended	1m 23s
ubuntu-gcc-build-codecov	3m 33s
ubuntu-gcc-build-perf-stats	1m 1s
	1h 13m 43s

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