(docs): Add doc on cleaning the repo. (#31120)

docs/react-v9/contributing/common-dev-snags.md

@@ -1,4 +1,16 @@
-// TODO #30642 A doc explaining common snags and trip hazards.
+### Cleaning your repo
 
-Assignee - Mason
-Reviewer - Sean
+Every once in a while you'll get build errors in node modules unrelated to your work. This usually happens when changes get checked into packages not related to the package you're working on.
+
+First, file an issue [here](https://github.com/microsoft/fluentui/issues/new/choose) to ensure the team can track this issue.
+
+Make sure all your desired changes are stashed or otherwise saved somewhere you can pull them back in.
+
+First try running:
+
+`yarn`
+
+If the issues still persist, clean your repo:
+
+`yarn cache clean`
+`yarn`