docs: restructure README into docs/ pages #4194

Job	Run time
checks / readme-snippets	14s
checks / pre-commit	32s
checks / test (3.11, lowest-direct, ubuntu-latest)	42s
checks / test (3.10, highest, ubuntu-latest)	50s
checks / test (3.10, highest, windows-latest)	2m 10s
checks / test (3.10, lowest-direct, windows-latest)	2m 44s
checks / test (3.11, highest, ubuntu-latest)	46s
checks / test (3.12, lowest-direct, ubuntu-latest)	47s
checks / test (3.12, lowest-direct, windows-latest)	3m 18s
checks / test (3.11, lowest-direct, windows-latest)	2m 29s
checks / test (3.12, highest, windows-latest)	3m 23s
checks / test (3.13, highest, ubuntu-latest)	43s
checks / test (3.10, lowest-direct, ubuntu-latest)	51s
checks / test (3.12, highest, ubuntu-latest)	52s
checks / test (3.13, lowest-direct, windows-latest)	2m 25s
checks / test (3.13, lowest-direct, ubuntu-latest)	46s
checks / test (3.11, highest, windows-latest)	2m 34s
checks / test (3.13, highest, windows-latest)	2m 31s
all-green	3s
	28m 40s

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