Add a page on when and how to report issues

[rgommers]

See user story at #42 (comment). Having good issue reporting guidelines is quite important, and our current content is insufficient probably.

What we have today:

1. https://github.com/numpy/numpy/blob/master/.github/ISSUE_TEMPLATE.md
2. https://numpy.org/devdocs/bugs.html
3. https://scipy.org/bug-report.html

It should end up in a sub-page or in the docs, and linked from Contribute, Community and Install probably.

[rgommers]

Leaving this for later, it doesn't fit on Community or Get Help pages. Could be a separate page, but can be done after website launch.