DOC: add `direnv` and `.envrc` instructions #689

Job	Run time
style / Determine skipped pre-commit hooks	7s
doc / nb / Execute and build documentation	2m 9s
doc / linkcheck / Check external links	1m 20s
style / Run local pre-commit hooks	38s
style / Run taplo through pre-commit	27s
style / Push changes	0s
doc / Upload to GitHub Pages	0s
	4m 41s

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