docs: add environment variables guide #1465

Job	Run time
checks / readme-snippets	11s
checks / pre-commit	33s
checks / test (3.10, locked, windows-latest)	2m 21s
checks / test (3.10, lowest-direct, windows-latest)	2m 36s
checks / test (3.11, lowest-direct, windows-latest)	2m 33s
checks / test (3.11, locked, ubuntu-latest)	49s
checks / test (3.11, lowest-direct, ubuntu-latest)	57s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 0s
checks / test (3.12, locked, windows-latest)	3m 59s
checks / test (3.12, locked, ubuntu-latest)	58s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 16s
checks / test (3.14, locked, windows-latest)	3m 15s
checks / test (3.11, locked, windows-latest)	2m 56s
checks / test (3.13, locked, windows-latest)	2m 54s
checks / test (3.14, locked, ubuntu-latest)	2m 24s
checks / test (3.14, lowest-direct, windows-latest)	3m 3s
checks / test (3.10, locked, ubuntu-latest)	55s
checks / test (3.10, lowest-direct, ubuntu-latest)	54s
checks / test (3.13, lowest-direct, windows-latest)	3m 17s
checks / test (3.13, lowest-direct, ubuntu-latest)	55s
checks / test (3.12, lowest-direct, windows-latest)	3m 45s
all-green	4s
	42m 23s

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