The Flink documentation build process is currently quite messy. These
changes move us to a new build process with proper dependency
handling. It assures that we all use the same dependency versions for
consistent build output. Also, it eases the automated building process
on other systems (like the ASF Buildbot). The goal was to make the
documentation build process easier and self-contained.

- use Ruby's Bundler Gem to install dependencies
- update README
- adapt Dockerfile
- add additional rules to .gitignore
- change default doc output path from /target to /content
(default path of the flink-web repository)

This closes #2033