docs(fleet): replace GitHub OAuth setup with GitHub App instructions #11116

Job	Run time
check-cross-refs	21s
check-generated-files	26s
lint (3.13) / make lint #3.13	21s
test (3.13) / make test #3.13	21s
links (3.13) / Documentation Build & Link Check (Python 3.13)	2m 9s
	3m 38s

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