Add 18.9.0 keyword documentation to repository. #9716

Job	Run time
build (ubuntu-latest)	1m 6s
testing (windows-latest, 3.10, 18.x, 1)	4m 49s
build (windows-latest)	2m 35s
testing (windows-latest, 3.12, 20.x, 2)	4m 1s
testing (ubuntu-latest, 3.10, 22.x, 3)	3m 0s
testing (ubuntu-latest, 3.11, 20.x, 4)	2m 41s
testing (windows-latest, 3.9, 20.x, 1)	4m 52s
testing (ubuntu-latest, 3.10, 18.x, 2)	2m 19s
testing (ubuntu-latest, 3.9, 18.x, 3)	2m 49s
testing (ubuntu-latest, 3.10, 20.x, 4)	2m 19s
testing (ubuntu-latest, 3.11, 20.x, 1)	2m 40s
testing (macos-latest, 3.11, 20.x, 1)	3m 34s
testing (macos-latest, 3.12, 20.x, 2)	3m 6s
testing (macos-latest, 3.10, 20.x, 3)	2m 45s
test-install (windows-latest, 3.11, 18.x)	6m 18s
test-install (ubuntu-latest, 3.9, 18.x)	4m 41s
test-install (ubuntu-latest, 3.11, 18.x)	4m 32s
test-install (ubuntu-latest, 3.11, 20.x)	4m 19s
	1h 2m 26s

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