docs: document search-metadata frontmatter for Algolia faceting

[Ryan-Amirthan]

Summary

Documents the search-metadata frontmatter block shipped in fern-api/fern-platform#14223. Replaces the draft in #6794, whose content didn't match the implementation: it claimed values must be strings and keys must be alphanumeric + hyphens, and it described the block as merely "stored on the record" without the record attribute name or the facet paths customers query by.

Three edits:

• customization/search.mdx: new ### Facet on custom page metadata under Integrating with Algolia (next to "Making search requests", since the attributes are only reachable through the Algolia API — not under "How search works", where the draft put it). Covers the frontmatter → search_metadata record attribute mapping, faceting by full path (facets: ["search_metadata.discovery.technology_tags"], facetFilters: [["search_metadata.audience:gardener"]]), the accepted value types (string/number/boolean/date/array/nested object), and the sanitizer's real constraints: keys containing : or . dropped, subtrees past depth 5 dropped, whole block dropped past 4KB. Also scopes it to markdown pages and notes it's absent from Fern's built-in filter bar.
• navigation/frontmatter.mdx: <ParamField path="search-metadata"> pointing at the canonical section.
• changelog/2026-09-01.mdx: new entry, tagged search, frontmatter.

fern check --warnings and Vale pass (remaining Vale suggestion on ## Integrating with Algolia is pre-existing).

Link to Devin session: https://app.devin.ai/sessions/973cdd79ce9a46699ac687616576445a
Open in Devin Desktop: https://app.devin.ai/desktop/session/973cdd79ce9a46699ac687616576445a?variant=devin
Requested by: @Ryan-Amirthan

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1788267295-search-metadata-docs.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/changelog/2026/9/1
• learn/docs/customization/search
• learn/docs/configuration/page-level-settings