Add stable category theory formalization #3909

Job	Run time
opam-build (latest, ubuntu-latest)	3m 55s
opam-build (supported, ubuntu-latest)	3m 51s
opam-build (dev, ubuntu-latest)	3m 23s
quick-build (dev, ubuntu-latest)	3m 4s
quick-build (latest, ubuntu-latest)	3m 8s
build (supported)	3m 52s
nix	2m 9s
build (dev, --warnings)	3m 28s
build (latest)	3m 19s
quick-build (supported, ubuntu-latest)	3m 39s
doc-timing	0s
doc-coqdoc	0s
doc-alectryon	0s
install	0s
doc-dep-graphs	0s
coqchk	0s
deploy-doc	0s
delete-artifacts	6s
	33m 54s

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