docs: expand README with install, onboard, and provider guide #14

Job	Run time
test	40s
build-and-push (ipc-bridge)	1m 16s
build-and-push (controller)	2m 37s
build-and-push (channel-telegram)	57s
build-and-push (channel-whatsapp)	57s
build-and-push (apiserver)	2m 43s
build-and-push (webhook)	2m 17s
build-and-push (channel-slack)	59s
build-and-push (channel-discord)	1m 0s
	13m 26s

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