Add detailed documentation for custom property editor migration to V14+

[kjac]

📋 Description

V14 introduces some significant changes for custom property editors. We attempt to handle these changes automatically as best we can, but this handling is based on assumptions... and apparently, we never documented those assumptions 🙈 so this PR amends that.

📎 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)

Ideally before the launch of V17.

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[kjac]

@eshanrnh This is only the V15 docs part ... I will triplicate it to V16 and V17 once you're happy with the V15 version (they'll be identical).

[eshanrnh]

@kjac, could you please add the article to the SUMMARY.md file so that I can preview and review it? 😀

[kjac]

@eshanrnh oops 🙈 done!

[eshanrnh]

Thanks for the PR, @kjac 🙌 The PR looks awesome 💪

I’ve suggested a few updates to improve clarity and consistency:

• Replaced “14+” with “14 and later” for style consistency.
• Removed “we” and fixed minor grammar issues and typos.
• Adjusted subheadings to be concise.
• Updated link text to match article title.

[kjac]

Good catches @eshanrnh 👍

I'll replicate the PR in the V16 and V17 docs, then you can merge it all in at once.

[kjac]

All done - I have marked the PR as ready for review. I'll leave the merging to you @eshanrnh 😄

[github-actions]

The Link Checker found broken links in your PR.
Please review the following list:

Broken links found in: 15/umbraco-cms/fundamentals/setup/upgrading/version-specific/README.md

Broken links found in: 16/umbraco-cms/fundamentals/setup/upgrading/version-specific/README.md

Broken links found in: 17/umbraco-cms/fundamentals/setup/upgrading/version-specific/README.md

[github-actions]

The Link Checker found broken links in your PR.
Please review the following list:

Broken links found in: 15/umbraco-cms/fundamentals/setup/upgrading/version-specific/README.md

Broken links found in: 16/umbraco-cms/fundamentals/setup/upgrading/version-specific/README.md

Broken links found in: 17/umbraco-cms/fundamentals/setup/upgrading/version-specific/README.md

[eshanrnh]

Thanks so much for updating the rest of the versions too, @kjac 🙌