docs: Add a guide on configuring ChannelPools for gRPC #18195

Job	Run time
generate-readme	1m 6s
owlbot-yaml	1m 1s
package_name_check	1m 9s
group_id_check_for_maps_libraries	8m 29s
gapic-bom	1m 47s
gitignore_correctness	1m 15s
owlbot-py	1m 10s
root-pom	1m 39s
	17m 36s

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