doc: explain GCS authentication options #6513

Job	Run time
build-rust-doc	6m 28s
build-c-doc	18s
build-cpp-doc	1m 47s
build-dav-server-opendalfs-doc	1m 8s
build-haskell-doc	10m 22s
build-java-doc	19s
build-lua-doc	9s
build-nodejs-doc	22s
build-object-store-opendal-doc	42s
build-ocaml-doc	5m 7s
build-python-doc	2m 20s
build-website	7m 10s
	36m 12s

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