docs: restructure README into docs/ pages #4189

Job	Run time
checks / readme-snippets	12s
checks / pre-commit	22s
checks / test (3.12, lowest-direct, ubuntu-latest)	50s
checks / test (3.13, lowest-direct, ubuntu-latest)	47s
checks / test (3.12, lowest-direct, windows-latest)	3m 14s
checks / test (3.10, highest, ubuntu-latest)	44s
checks / test (3.12, highest, windows-latest)	3m 34s
checks / test (3.10, lowest-direct, ubuntu-latest)	45s
checks / test (3.12, highest, ubuntu-latest)	53s
checks / test (3.11, highest, windows-latest)	2m 32s
checks / test (3.13, lowest-direct, windows-latest)	2m 23s
checks / test (3.13, highest, windows-latest)	2m 31s
checks / test (3.10, lowest-direct, windows-latest)	2m 16s
checks / test (3.11, lowest-direct, ubuntu-latest)	45s
checks / test (3.13, highest, ubuntu-latest)	48s
checks / test (3.10, highest, windows-latest)	2m 21s
checks / test (3.11, highest, ubuntu-latest)	48s
checks / test (3.11, lowest-direct, windows-latest)	2m 25s
all-green	3s
	28m 13s

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