docs: Improve GitHub token setup documentation in UI guide

[neubig]

The current documentation about GitHub token setup in the GUI mode guide is minimal and could be improved to help users better understand the process.

Needed improvements:

1. Add step-by-step instructions for setting up a GitHub token locally:

   • How to access the token settings in the UI
   • Required token scopes and permissions
   • How to enter and save the token

2. Add information about organizational token policies:

   • Note that organizational repositories may require additional setup
   • Link to GitHub's documentation about setting organizational token policies https://docs.github.com/en/organizations/managing-programmatic-access-to-your-organization/setting-a-personal-access-token-policy-for-your-organization
   • Instructions for checking if token policies are blocking access

3. Add troubleshooting section:

   • Common issues with token setup
   • How to verify token is working correctly
   • What to do if token is not being recognized

4. Clarify OAuth flow for online mode:

   • Explain what permissions are requested
   • How to review and approve access
   • What to do if authentication fails

This will help users better understand how to set up GitHub integration in both local and online modes, and how to handle organizational requirements.

[github-actions]

OpenHands started fixing the issue! You can monitor the progress here.

[github-actions]

An attempt was made to automatically fix this issue, but it was unsuccessful. A branch named 'openhands-fix-issue-5162' has been created with the attempted changes. You can view the branch here. Manual intervention may be required.