Document custom heading IDs

[mintlify]

Summary

• Added documentation for the new {#custom-id} syntax that lets users set custom anchor IDs on headings (levels 1-4)
• Updated the linking guide to reference custom anchor links alongside auto-generated ones

Headings now support a {#custom-id} suffix to override the auto-generated anchor link. This helps users keep anchors stable when renaming headings, create shorter anchor links, and avoid collisions.

Triggered by mintlify/mint@b5cf3f50.

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Note

Low Risk
Documentation-only change that adds guidance for a new heading syntax; no runtime code or behavior is modified.

Overview
Documents the {#custom-id} suffix for headings so authors can override auto-generated anchor IDs, including guidance on supported heading levels, slug sanitization, and how duplicate IDs are disambiguated.

Updates the linking guide to include a Custom anchor links section and cross-links to the new Format text documentation.

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