Remove all editormodel articles, redirects etc. for 15+

[kjac]

📋 Description

As a follow-up for #7443, this PR removes all mentions of "Editor Model" notifications for all V15+ docs.

While doing this, I found a small oversight from #7443 - the V16 notifications reference still links to the editormodel-notifications article:

...I have removed that as well.

📎 Related Issues (if applicable)

None.

✅ 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)

V14+ 😆

Deadline (if relevant)

None.

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[eshanrnh]

Thanks for updating the articles quickly, @kjac 🙌