docs: add multi-tenancy guide, example server, and OAuth e2e tests #51

Job	Run time
checks / pre-commit	31s
checks / readme-snippets	15s
checks / test (3.10, locked, ubuntu-latest)	53s
checks / test (3.10, lowest-direct, windows-latest)	2m 47s
checks / test (3.11, lowest-direct, windows-latest)	2m 21s
checks / test (3.12, locked, ubuntu-latest)	58s
checks / test (3.10, locked, windows-latest)	2m 28s
checks / test (3.13, locked, ubuntu-latest)	46s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 1s
checks / test (3.11, locked, ubuntu-latest)	52s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 21s
checks / test (3.14, lowest-direct, windows-latest)	2m 17s
checks / test (3.10, lowest-direct, ubuntu-latest)	55s
checks / test (3.12, lowest-direct, windows-latest)	3m 52s
checks / test (3.13, lowest-direct, ubuntu-latest)	55s
checks / test (3.11, lowest-direct, ubuntu-latest)	51s
checks / test (3.11, locked, windows-latest)	2m 16s
checks / test (3.14, locked, ubuntu-latest)	2m 18s
checks / test (3.12, locked, windows-latest)	3m 5s
checks / test (3.13, lowest-direct, windows-latest)	2m 34s
checks / test (3.13, locked, windows-latest)	2m 15s
checks / test (3.14, locked, windows-latest)	2m 42s
all-green	3s
	38m 16s

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