Add comprehensive guide on hiding UI elements

[mintlify]

Created a new comprehensive guide that consolidates all methods for hiding UI elements in Mintlify documentation, including navigation elements, API sections, and custom CSS approaches. The guide provides practical examples, best practices, and covers everything from hiding pages and tabs to controlling API endpoint visibility and using custom CSS selectors.

Files changed:

• customize/hiding-ui-elements.mdx - New comprehensive guide on hiding UI elements
• docs.json - Added new guide to Customize navigation group

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Note

Low Risk
Documentation-only changes plus a nav entry update; minimal risk beyond potential broken links/formatting issues.

Overview
Adds a new customize/hiding-ui-elements.mdx guide consolidating ways to control documentation UI visibility, including hiding navigation items (pages/groups/tabs/anchors/dropdowns), OpenAPI endpoint visibility (x-hidden/x-excluded and explicit pages lists), and CSS-based hiding with common selectors and cautions.

Updates docs.json to include the new guide under the Customize section so it appears in the published docs navigation.

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