add backend intro and how-to diagram #13308

Job	Run time
detect ci trigger	2s
Event File	2s
ubuntu-latest py3.9	8m 38s
ubuntu-latest py3.12	10m 29s
macos-latest py3.9	7m 7s
macos-latest py3.12	8m 34s
windows-latest py3.9	13m 40s
windows-latest py3.12	17m 9s
ubuntu-latest py3.9 bare-minimum	2m 8s
ubuntu-latest py3.9 min-all-deps	7m 22s
ubuntu-latest py3.11 all-but-dask	7m 20s
ubuntu-latest py3.12 flaky	1m 51s
	1h 24m 22s

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