Release 2.6.0: Improve search results and FileSet documentation

- Fixes the icons used when returning a search; makes the icons linked to the search result:

http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-r2.6.0_fix_search_links/en/search.html?q=FileSet&check_keywords=yes&area=default

- Changes FileSet description to clarify the nature of Input and Output formats for FileSets:

Staged at: http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT-r2.6.0_fix_search_links/en/developers-manual/building-blocks/datasets/fileset.html