Add "Returning to freeCodeCamp" guide for updating existing forks

## Description

I'd like to propose adding a new guide called "Returning to freeCodeCamp" that would help contributors who are coming back after some time away. This guide would closely follow the structure of the existing "Set up freeCodeCamp" documentation but focus specifically on updating an existing fork.

## Problem Statement

Contributors who have previously forked and cloned the freeCodeCamp repository often face confusion when returning after a period of inactivity. Their fork may be outdated, and they need clear instructions on how to:
- Update their fork with the latest changes from the main repository
- Resolve any potential conflicts
- Ensure their local environment is properly updated

## Proposed Solution

Create a new guide that covers:
1. **Checking fork status** - How to verify if your fork is behind the main repository
2. **Updating the fork** - Step-by-step instructions for syncing with upstream
3. **Updating local clone** - Pulling the latest changes to your local machine
4. **Handling conflicts** - Basic guidance on resolving merge conflicts if they arise
5. **Refreshing dependencies** - When and how to update npm packages
6. **Verifying the setup** - Quick tests to ensure everything is working correctly

## Benefits

- Reduces friction for returning contributors
- Complements the existing "Set up freeCodeCamp" guide
- Helps maintain a healthy contributor ecosystem
- Reduces duplicate questions in Discord/forums about updating forks

## Implementation

This guide would be added to the existing documentation structure and linked from relevant places like:
- The main README
- The "Set up freeCodeCamp" guide (as a "Returning?" callout)
- Contributing guidelines

I'm happy to help draft this guide if the team agrees it would be valuable!