Add links to downloads and docs of Gerrit v2.16.9
Add links to downloads and docs of Gerrit v2.16.9

Change-Id: Idb72b8c89f47715defffaedee90a7f33c6603b00

GerritCodeReview/homepage,GerritCodeReview/homepage,GerritCodeReview/homepage,GerritCodeReview/homepage,GerritCodeReview/homepage