Backport PR #5692: Update notebook.rst fixing broken reference to notebook examples readme

Since the notebook directory doesn't contain a readme anymore, updating to point to the index notebook rendered via nbviewer
Backport PR #5693: Update parallel_intro.rst to fix a broken link to examples

fixing broken link to examples directory and updating the github link to point to the nbviewer index page
Backport PR #5699: don't use common names in require decorators

causes kwarg collisions if the depended-upon function has the same name.

closes #5698