Docs: Improve the create pull request guide for contributing (#34799)

- add extra section about frontend unit test

Co-authored-by: Marcus Olsson <marcus.olsson@hey.com>
Co-authored-by: Hugo Häggmark <hugo.haggmark@grafana.com>
Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

contribute/create-pull-request.md

@@ -43,6 +43,11 @@ Pull requests for Redux contributions must:
 - Not contain code that mutates state in reducers or thunks.
 - Not contain code that accesses the reducers state slice directly. Instead, the code should use state selectors to access state.
 
+Pull requests that add or modify unit tests that are written in Jest must adhere to these guidelines:
+
+- Don't add snapshots tests. We are incrementally removing existing snapshot tests, we don't want more.
+- If an existing unit test is written in Enzyme, migrate it to RTL (React Testing Library), unless you’re fixing a bug. Bug fixes usually shouldn't include any bigger refactoring, so it’s ok to skip migrating the test to RTL.
+
 ### Backend-specific guidelines
 
 Please refer to the [backend style guidelines](/contribute/style-guides/backend.md).