Add link to download PDF of documentation

This commit adds a sidebar box with a link to the
Fuel Development documentation in the form of a
PDF.  The PDF document will be built automatically
by Jenkins when the docs are built.

Change-Id: I45a07043340b50c24dadee8143a3e475c239d545
Implements: blueprint fuel-web-docs-dev-env-restructure
Merge "Add link to download PDF of documentation"