Documentation: add a TL;DR summary

The Java 13 recipe is the first notable example command, which makes
it seem the recommended recipe for hurried readers.

Change-Id: Idf35ca549350d66172e2ea22d6adfe61646ed955

Documentation/dev-bazel.txt

@@ -1,6 +1,17 @@
 :linkattrs:
 = Gerrit Code Review - Building with Bazel
 
+[[summary]]
+== TL;DR
+
+If you have the prerequisites, running
+
+```
+  $ bazel build gerrit
+```
+
+should generate a .war file under `bazel-bin/gerrit.war`.
+
 [[installation]]
 == Prerequisites