Build `data/toc.yml`

[darribas]

This needs to be on the fly with a structure that represents the notebooks pull into content (see #46).

In addition, the data/toc.yml file also can have a base structure that sets up parts of the book that are static and do not depend on content from other repos.

[darribas]

This is done now, for the most part, in b3aad99fd2c5308ce14dcd74489ddc7418c9fafb. However, I'm leaving this open as the README files, which are used as the landing page for each package chapter, should still be improved (e.g. the badges render too big).

One solution could be picking only a standardized part of the entire README, or picking the description of the package from another part.

[darribas]

I've opened an issue to try to clarify the size of the badge image here