Add btree_gist dependency in docs #575

Job	Run time
sdist	8s
linux (x86_64)	1m 5s
changes	4s
linux (x86)	1m 4s
macos (x86_64)	36s
windows (x64)	2m 37s
linux (aarch64)	1m 20s
windows (x86)	3m 3s
linux (armv7)	1m 31s
macos (aarch64)	38s
linux (s390x)	1m 19s
linux (ppc64le)	1m 14s
Build and push base pyrust	0s
Build and push base postgres image	0s
test	0s
	14m 39s

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