Add link to Gerrit builds on CI

To make the Gerrit builds more discoverable over
the internet, it is better to add a link from the Gerrit
home page.

This allows the CI to be indexed by Google Search engine
and people to easily find pre-built version of their plugins.

Change-Id: I5a4b4dfc25df572d7c0fb17b72dc46d05b31de0b

navbar.md

diff --git a/navbar.md b/navbar.md
index 96a3cd7..001caa7 100644
--- a/navbar.md
+++ b/navbar.md
@@ -7,6 +7,7 @@
 * [Wiki](https://gerrit.googlesource.com/homepage/+/md-pages/docs/)
 * [Source](https://gerrit.googlesource.com/gerrit/)
 * [Reviews](https://gerrit-review.googlesource.com/#/q/status:open+project:gerrit)
+* [Builds](https://gerrit-ci.gerritforge.com)
 
 [home]: /index.md
 [logo]: /images/diffy45.png