Update FAQ with upgrade failure troubleshooting

[danlister]

This PR adds a new FAQ entry to the Resolve issues quickly and efficiently section.
The new section documents a scenario where project upgrades (e.g. Standard → Pro) can fail due to excessively long file or folder paths within the repository. It explains the symptoms, cause, and recommended fix to help users identify and resolve the issue quickly.

📎 Related Issues (if applicable)

N/A

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• Code blocks are correctly formatted.
• Sentences are short and clear (preferably under 25 words).
• Passive voice and first-person language (“we”, “I”) are avoided.
• Relevant pages are linked.
• All links work and point to the correct resources.
• Screenshots or diagrams are included if useful.
• Any code examples or instructions have been tested.
• Typos, broken links, and broken images are fixed.

Product & Version (if relevant)

Umbraco Cloud

Deadline (if relevant)

As soon as possible 😄

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[eshanrnh]

Thanks for the PR, @danlister 🙌 The PR looks great. I added two suggestions to fix vale errors.