[FLINK-1282] [docs] Update layout of docs

This closes #235.
[FLINK-1282] [docs] Added {{ site.baseurl }} to css and image paths
[FLINK-1282] [docs] Make Flink logo link back to frontpage

This closes #237.
[docs] Fix paths to work with local builds

Local builds of the docs without a web server (e.g. ./build_docs.sh -p)
were not working. This commit fixes this.