Add or expand docs.rst

Change-Id: I18d1529088beb41b37be00a77ba8999f6b6f810a
diff --git a/pw_varint/BUILD.gn b/pw_varint/BUILD.gn
index d1aa98c..9b79a5c 100644
--- a/pw_varint/BUILD.gn
+++ b/pw_varint/BUILD.gn
@@ -12,6 +12,7 @@
 # License for the specific language governing permissions and limitations under
 # the License.
 
+import("$dir_pw_docgen/docs.gni")
 import("$dir_pw_unit_test/test.gni")
 
 config("default_config") {
@@ -39,3 +40,7 @@
     "varint_test.cc",
   ]
 }
+
+pw_doc_group("docs") {
+  sources = [ "docs.rst" ]
+}