Documentation: Make steps for addressing #140 more prominent in docs

[astrowonk]

Problem

I needed to run the steps in #140 to fix the nbclassic extension configurator. While these are listed in the docs but they were listed in Contribute->Development, which took me a long time to find. I was looking in User Documentation and in the section on extensions.

Proposed Solution

These steps are needed for all users who want to use the configurator in nbclassic, not only those who want to develop or contribute. I suggest moving the list of known issues (and their fixes) prominently to User Documentation., or perhaps a known issues page in the table of contents under user documentation. Thanks!

Oh, and the 0.6.4 of the configurator is on pypi, so the steps could be updated to skip installing from github and just upgrade from pypi.

[krassowski]

I think it would be reasonable o move it, feel welcome to submit a PR!