[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 apache#878.

Author: Baunsgaard

.gitignore

@@ -37,6 +37,10 @@ buildNumber.properties
 .metadata
 .cache*
 
+# Documentation artifacts
+.jekyll-cache/
+_site/
+
 # Python artifacts
 src/main/python/systemds/systemds-java/
 src/main/python/systemds.egg-info/