Final touch to the documentation after update to 2.0

[WeisLeDocto]

This PR brings a final touch to the documentation of Crappy, that has already been greatly completed and enhanced by #52. As detailed in #66, this PR contains the following improvements:

• Use the kbd directive when describing a keyboard combination
• Use the versionadded, versionchanged and versionremoved directives for signaling the version when a class, method, or argument was added, changed, or removed
• Add sectionauthor directives for signaling the author(s) of documentation sections
• Add the link to numpy, matplotlib, and psutil documentation in intersphinx
• Add collapse menus to the documentation for hiding large sections of code

The changes will be included in version 2.0.1.
This PR closes #66.