Add a Sphinx extension to copy over Retina images.

Sphinx only actually copies over images that are used in documents. This
means that while we can dump @2x copies of images into _static, they
won't be used.

This extension hooks into the environment generation and html page
collection events, looking for @2x equivalents for all images that are
in use. It then adds them to the list of images set to be copied into
_images.
Remove documentation on installing nightlies.

Remove documentation on installing nightlies from the "Installing Development
Releases" section of the manual, since nightlies are no longer built. Also
clarify the bit about differentiating from the installation docs, meaning
standard (production) installation docs.

Testing Done:
Visually inspected generated HTML docs.

Bugs closed: 3252

Reviewed at https://reviews.reviewboard.org/r/5514/