Added documentation on new dictionary value search config

[Nis-Knowit]

📋 Description

This PR adds documentation for the new dictionary value search feature introduced in umbraco/Umbraco-CMS#21200.

The feature allows searching dictionary items by their translation values in addition to keys. It's an opt-in configuration setting that's disabled by default to preserve backward compatibility and performance.

Changes:

• Added new configuration documentation: 17/umbraco-cms/reference/configuration/dictionarysettings.md
• Updated configuration index to include Dictionary Settings
• Documented the UseDictionaryValueSearch configuration option

📎 Related Issues (if applicable)

Related to code PR: umbraco/Umbraco-CMS#21200

✅ 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)

• Product: Umbraco CMS
• Version: 17

Deadline (if relevant)

Should be published when umbraco/Umbraco-CMS#21200 is merged and released.

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[sofietoft]

Thanks for the PR @Nis-Knowit ! 🙌
I'll tag this for pending release, so we can get it merged once the feature is added to the product.