Document .ipynb_checkpoints directory

[JasonWeill]

In investigating #6654, we discovered that the docs for Jupyter Notebook don't mention the checkpoints directory, .ipynb_checkpoints. We occasionally get bugs about this, such as about people unclear whether it should be visible or unaware that it could help them recover an accidentally-deleted notebook.

Add info about checkpoints to the docs.

[kostyafarber]

What exactly would we want to describe about this directory in the docs?

Any place in the docs where we would want to put it?

Happy to try write something up about it if it's simply just a directory that stores notebooks for recovery.

(I'm not aware however of how notebook does this)

[jtpio]

What exactly would we want to describe about this directory in the docs?

Probably something that explains why such folder exist and why it can be useful?

Any place in the docs where we would want to put it?

Maybe a new section in the user facing docs? https://jupyter-notebook.readthedocs.io/en/latest/user-documentation.html

[akashthemosh]

take