DOC: add `direnv` and `.envrc` instructions #678

Job	Run time
style / Determine skipped pre-commit hooks	8s
doc / nb / Execute and build documentation	2m 24s
doc / linkcheck / Check external links	1m 43s
style / Run taplo through pre-commit	20s
style / Run local pre-commit hooks	37s
style / Push changes	0s
doc / Upload to GitHub Pages	0s
	5m 12s

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