Possible solution to combine all package docs

[bdice]

We’ve been wanting to combine API docs and not deal with the “subpackage” approach for a long time. I would propose the following, but I am not sure if the solution below is viable. Related to #75 ...and probably many other issues.

There might be some way we could make the signac-docs repo include git submodules, or clone signac/flow/dashboard in the Makefile, AND couple this with a GitHub web hook that would trigger rebuilds whenever commits are made to the main branches of signac/flow/dashboard.