pw_unit_test: Start documentation

This change adds some initial documentation for the unit test module.

Change-Id: Icc9eaa67c11766af8eb3921c9079771efe152af8
diff --git a/pw_unit_test/BUILD.gn b/pw_unit_test/BUILD.gn
index 4283c14..d849614 100644
--- a/pw_unit_test/BUILD.gn
+++ b/pw_unit_test/BUILD.gn
@@ -13,6 +13,7 @@
 # the License.
 
 import("$dir_pw_build/pw_executable.gni")
+import("$dir_pw_docgen/docs.gni")
 import("$dir_pw_unit_test/test.gni")
 
 config("default_config") {
@@ -60,6 +61,10 @@
   sources = [ "simple_printing_main.cc" ]
 }
 
+pw_doc_group("docs") {
+  sources = [ "docs.rst" ]
+}
+
 # Library providing an event handler which logs using pw_log.
 if (dir_pw_log_backend != "") {
   source_set("logging_event_handler") {