Mention possible location of configuration files in documentation

[astrojuanlu]

This was mentioned in the repo2docker docs, but not on the binder ones.

In addition, I used this opportunity to enable intersphinx in the repository.

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[astrojuanlu]

Sphinx 4.0.3 got installed, which doesn't have the add_stylesheet method anymore.

On the other hand:

Traceback (most recent call last):
  File "federation/gen_federation.py", line 3, in <module>
    import pandas as pd
ModuleNotFoundError: No module named 'pandas'

Unsure how to continue 🤔

[astrojuanlu]

Oh I see, a couple of extensions pull sphinx>=2. Easy fix.

[astrojuanlu]

Rendered version: https://mybinder--236.org.readthedocs.build/en/236/config_files.html#configuration-files

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Congrats on your first merged pull request in this project! :tada: Thank you for contributing, we are very proud of you! :heart:

[choldgraf]

looks great - thanks very much! :-)