add backend intro and how-to diagram #13168

Job	Run time
Event File	2s
detect ci trigger	5s
ubuntu-latest py3.9	7m 56s
ubuntu-latest py3.12	9m 35s
macos-latest py3.9	5m 28s
macos-latest py3.12	6m 41s
windows-latest py3.9	15m 57s
windows-latest py3.12	20m 57s
ubuntu-latest py3.9 bare-minimum	1m 36s
ubuntu-latest py3.9 min-all-deps	7m 2s
ubuntu-latest py3.11 all-but-dask	6m 23s
ubuntu-latest py3.12 flaky	1m 22s
	1h 23m 4s

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