Adopt a monorepo

[mbostock]

Now that all the documentation is in this repo, we can’t make documentation changes alongside PRs in other repos (such as d3-time). We should make this a monorepo so that we can update the documentation at the same time and to make things easier to maintain.

[mbostock]

Using separate repos is still desirable because it lets us use GitHub’s releases system (and tags for releases). We have thinking about breaking apart the documentation into the respective repos, and having some build process here which collects them so that it can still be served as a separate site.