fix(docsite): add example and usage guidance for BaseTypeahead (#2679)

[mohitWeb-lab]

Summary

The BaseTypeahead docsite page had no meaningful example and no usage guidance, leaving an empty Examples section and no playground link. It was also unclear whether the component was intended for direct use or was a primitive backing higher-level components.

Cause

BaseTypeahead had no usage section in its doc and no example blocks registered under exampleFor: 'BaseTypeahead'. The component is a headless primitive (subComponentOf: 'Typeahead', isHiddenFromOverview: true) used internally by Typeahead and Tokenizer, so no examples were ever authored for it. Without guidance, the page silently showed nothing.

Fix

• Added a usage section to BaseTypeahead.doc.mjs with a description matching the existing component description, 3 Do / 2 Don't best practices aligned to its "no wrapper, no border, no token rendering" nature, and relatedComponents: ['Typeahead', 'Tokenizer'] to surface links to the correct higher-level components.
• Added a BaseTypeaheadCustomSearch example block showing BaseTypeahead embedded inside a custom-styled HStack wrapper with its own border and search icon, demonstrating the anchorRef pattern and createStaticSource — the exact composition use case the component is designed for.

Closes #2679

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