Add API docs in markdown

The objective is to have a single file in docs/api.md that shows the summary of the API. Doxygen with all the HTML, sub-menus, etc. is overkill for our needs.

This requires doxybook2: https://github.com/matusnovak/doxybook2

To rebuild it, we need doxygen and doxybook2 in the path and set -DBUILD_DOCUMENTATION=ON