[DOC] - Explain jovyan user directory

[ericdatakelly]

Considering the perspective of someone who may not be familiar with JupyterHub, or maybe even Jupyter, the docs should explain what the jovyan user directory is for.

This can be a problem when setting up ssh keys for GitHub. ssh-keygen puts the new key in the /home/jovyan/.ssh directory. However, someone not familiar with the jovyan directory will look for the key in the /home/<user>/.ssh directory (but it's not there).

Related: #1076

[trallard]

Adding these resources for reference:

• https://docs.jupyter.org/en/latest/community/content-community.html#what-is-a-jovyan
• https://discourse.jupyter.org/t/what-is-with-the-weird-jovyan-user/1673

[costrouc]

I'm wondering how this is happening since jovyan no longer exists in the docker images (from #1072). Thanks for the issues @ericdatakelly!