Create dev build infrastructure

[handrews]

I had a check fail on #4188 on v3.2-dev and realized that there's no way to build without copying things over to the publish location (and even then something went weird when it was trying to build the old versions b/c the dev branches aren't set up for that).

We should be able to build just src/oas.md for testing purposes on dev and all branches derived from dev.

This change should go into dev and be merged/cherry-picked out from there.

If Arazzo and Overlay are going to use the same system, this might also be a good time to start building a shared system instead of continuing to patch things in triplicate.