Add coordinate system example

[ManonMarchand]

This PR implements:

• minor fixes to the example notebook external_neighbors.ipynb following API changes in both mocpy and numpy
• a tiny docstring modification in healpix_to_skycoord to specify that the input should be in nested configuration + added the output for the example
• add jupyter-sphinx as an extension for pretty examples
• add a long example in the docs on coordinate system changes
• fix a broken path in benchmark workflow