docs: add multi-tenancy guide, example server, and OAuth e2e tests #50

Job	Run time
checks / readme-snippets	16s
checks / pre-commit	18s
checks / test (3.10, locked, ubuntu-latest)	47s
checks / test (3.10, locked, windows-latest)	2m 34s
checks / test (3.11, lowest-direct, ubuntu-latest)	54s
checks / test (3.13, lowest-direct, windows-latest)	2m 19s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 1s
checks / test (3.12, lowest-direct, windows-latest)	3m 21s
checks / test (3.13, lowest-direct, ubuntu-latest)	50s
checks / test (3.10, lowest-direct, windows-latest)	2m 28s
checks / test (3.10, lowest-direct, ubuntu-latest)	57s
checks / test (3.11, locked, ubuntu-latest)	52s
checks / test (3.12, locked, windows-latest)	3m 0s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 12s
checks / test (3.11, lowest-direct, windows-latest)	2m 55s
checks / test (3.12, locked, ubuntu-latest)	55s
checks / test (3.13, locked, windows-latest)	1m 59s
checks / test (3.11, locked, windows-latest)	2m 19s
checks / test (3.14, locked, windows-latest)	3m 40s
checks / test (3.13, locked, ubuntu-latest)	50s
checks / test (3.14, locked, ubuntu-latest)	2m 15s
checks / test (3.14, lowest-direct, windows-latest)	2m 36s
all-green	5s
	38m 23s

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