Document custom heading IDs syntax

[mintlify]

Summary

• Added a new "Custom heading IDs" section to the text formatting page documenting the {#custom-id} syntax for overriding auto-generated heading anchors
• Updated the linking guide to mention custom heading IDs as an option when explaining how anchor links are generated

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Note

Low Risk
Low risk documentation-only change; no product logic or behavior is modified.

Overview
Adds documentation for custom heading IDs using {#custom-id} on create/text.mdx, including examples, sanitization behavior, supported heading levels, and duplicate-ID suffixing.

Updates guides/linking.mdx to mention custom IDs as an override to auto-generated anchors and links readers to the new section.

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