Add notice to current fe guide about new fe guide

doc/development/fe_guide/index.md

 # Frontend Development Guidelines
 
+> **Notice:**
+We are currently in the process of re-writing our development guide to make it easier to find information. The new guide is still WIP but viewable in [development/new_fe_guide](../new_fe_guide/index.md)
+
 This document describes various guidelines to ensure consistency and quality
 across GitLab's frontend team.