BIGTOP-1156. Fix formating in our website.

The pdf plugin is defined in our pom but do not work because of some malformed xdoc files.
This commit does the following:
* Update the version of the pdf plugin
* Fix xdoc files so a pdf of our website can be generated

This was tested by typing: mvn pdf:pdf
BIGTOP-1157. Add a new Apache Bigtop user guide

This is the first step in introducing a user guide for Apache Bigtop.
This commit adds all the necessary files to build such guide. It builds
a PDF and html version as part of the "maven site" command