Link API docs to libtopotoolbox docs

[wkearn]

We should explain in greater detail what each function is doing, and we might want to link to the libtopotoolbox documentation to avoid duplicating this information. We could obviously just use html links between the documentation, but those might break easily, so require manually synchronizing the pytopotoolbox docs with the libtopotoolbox docs. Maybe there is a better way to link between them that uses the common Sphinx infrastructure?