Added support indication and how to for downgrades and re-running migrations

[AndyButland]

📋 Description

Although downgrades and re-rerunning migrations aren't supported in Umbraco, there can be cases where they are safe, and a more advanced user can determine this and decide they want to proceed with it.

In those cases, it's useful to have documented how to do this and what the caveats are, that this PR provides.

✅ 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)

CMS

Deadline (if relevant)

Anytime (though please wait for a CMS team member to review too).

[kjac]

Looks good to me 👍

[eshanrnh]

Thanks for the PR, @AndyButland 🙌 Looks great.

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Broken links found in: 16/umbraco-engage/developers/personalization/implement-your-own-segment-parameters.md

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[github-actions]

The Link Checker found broken links in your PR.
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Broken links found in: 16/umbraco-engage/developers/personalization/implement-your-own-segment-parameters.md

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⚓ Anchor not found → 16/umbraco-engage/developers/personalization/implement-your-own-segment-parameters.md#id-2.-angularjs-definition | Cannot find fragment

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