Add download and documentation links for 2.15.13
Add download and documentation links for 2.15.13

Change-Id: I15539500e4a7ba3adad946dc5f5c19d39e6f79ba

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