Add GCS migration guide #1234

Job	Run time
get_available_profiles	9s
merge_client_pr	0s
close_client_pr	0s
update_client_pr (android-uploader, api.video-android-uploader)	43s
update_client_pr (android, api.video-android-client)	1m 5s
update_client_pr (csharp, api.video-csharp-client)	26s
update_client_pr (go, api.video-go-client)	37s
update_client_pr (java, api.video-java-client)	50s
update_client_pr (nodejs, api.video-nodejs-client)	53s
update_client_pr (php, api.video-php-client)	23s
update_client_pr (python, api.video-python-client)	31s
update_client_pr (swift5-uploader, api.video-swift-uploader)	26s
update_client_pr (swift5, api.video-swift-client)	23s
generate_documentation	24s
	6m 50s

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