Update ESO docs

Hey!

I plan to make some updates to the astroquery.eso docs (tagging @juanmcloaiza).

Is it a preference to have the docs in a flat structure page ([as is now](https://astroquery.readthedocs.io/en/latest/eso/eso.html)), or with heierarchy like with [MAST](https://astroquery.readthedocs.io/en/latest/mast/mast.html)? I can see the benefits of having either, particularly for the latter when we add more functionality.

-- Ash

Compare the current ESO page at astroquery.readthedocs.io/en/latest/eso/eso.html with the MAST hierarchy at astroquery.readthedocs.io/en/latest/mast/mast.html. First resolve the preferred documentation structure with the maintainers; done means the ESO docs follow the agreed structure and accurately cover the existing functionality.

Written by the indexing model from the issue text.