DOC: fix links to matplotlib, notebook docs

I've added a link to pandas, made all matplotlib links point to
matplotlib.org, and fixed a link in the notebook help menu to point to
our new notebook.html, now that htmlnotebook.html is a redirect.

Whoever ends up merging this PR should backport it to 1.x.
Merge pull request #4022 from ivanov/doc-links

DOC: fix links to matplotlib, notebook docs