Docs: Add and populate troubleshooting section for triggering pipelines

doc/ci/triggers/README.md

@@ -5,7 +5,7 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 type: tutorial
 ---
 
-# Triggering pipelines through the API
+# Triggering pipelines through the API **(CORE)**
 
 Triggers can be used to force a pipeline rerun of a specific `ref` (branch or
 tag) with an API call.
@@ -270,3 +270,11 @@ Old triggers, created before GitLab 9.0 are marked as legacy.
 Triggers with the legacy label do not have an associated user and only have
 access to the current project. They are considered deprecated and might be
 removed with one of the future versions of GitLab.
+
+## Troubleshooting
+
+### '404 not found' when triggering a pipeline
+
+A response of `{"message":"404 Not Found"}` when triggering a pipeline might be caused
+by using a Personal Access Token instead of a trigger token. [Add a new trigger](#adding-a-new-trigger)
+and use that token to authenticate when triggering a pipeline.