[FLINK-963] Initial import of documentation from website into source code

This is [FLINK-963](https://issues.apache.org/jira/browse/FLINK-962).

This is the initial import of the current documentation from http://www.stratosphere.eu/docs/0.5. It is not finished yet, but I chose to do the PR asap in order to get some help from others to speed this up. I tried to fix everything on my own, but it is simply too much for me right now. I managed to fix up most parts, but quick starts and programming guides are still under construction (see blow).

To make it clear: the layout stuff is not finished and just very simple. But the great thing about this change is that layout and content are now completely independent of each other. So we can continue with the conent, while I or someone else continue on the layout.

All documentation pages are organized in a flat hierarchy in the new `docs` directory (this makes links between doc pages way simpler, e.g. `{{ site.baseurl }}/docs/{{ site.version}}/programming_guides/java_api.html` vs. ``java_api_guide.html`). In contrast to the old documentation, we can just rely on Markdown ([GitHub flavored](http://github.github.com/github-flavored-markdown)) for the pages. The headings in the document are used to generate a per-page outline.

The provided `docs/build_docs.sh` script generates the docs to `docs/target` and can be run in preview mode `docs/build_docs.sh -p` to start a local webserver on port 4000.

What needs to be done:
1. Trim pages to plain Markdown
2. Fix links to GitHub
3. Fix references to old ``site`` variables
4. Fix images
5. Improve the layout

Thanks to some upcoming automation scripts by @rmetzger, we will be able use the same Markdown files on the upcoming website (with different layout).

I would like to merge this, if everyone agrees with the general setup and then divide the remaining work.