rocksdb hit the problem that nuclide had.

https://github.com/facebook/nuclide/issues/789
https://github.com/facebook/nuclide/pull/793

This is the initial commit with the templates necessary to have our RocksDB user documentation hosted on GitHub pages.

Ensure you meet requirements here: https://help.github.com/articles/setting-up-your-github-pages-site-locally-with-jekyll/#requirements

Then you can run this right now by doing the following:

```
% bundle install
% bundle exec jekyll serve --config=_config.yml,_config_local_dev.yml
```

Then go to: http://127.0.0.1:4000/

Obviously, this is just the skeleton. Moving forward we will do these things in separate pull requests:

- Replace logos with RocksDB logos
- Update the color schemes
- Add current information on rocksdb.org to markdown in this infra
- Migrate current Wodpress blog to Jekyll and Disqus comments
- Etc.