[MINOR] .gitignore for documentation build
[MINOR] .gitignore for documentation build

- Ignores Jekyll static build files and cache
  1. `.jekyll-cache`
  2. `_site` - html, css, js and media files
- This way one could cleanly work on documentation
  and the project.

Closes #878.

apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml