[DOC] Refocus filters.md to filter semantics and link Search docs

[tazarov]

Context

Filters docs currently include an interactive playground that also exposes non-filter search options in an advanced section.

Goal

Refocus docs/core/filters.md so it stays strictly about filtering semantics.

Scope

• Keep filter operator/schema coverage centered on:
  • metadata where
  • document where_document
• Move or de-emphasize search-only controls/content (ranking, grouping, batch, pagination) out of Filters docs.
• Ensure interactive examples on Filters page emphasize filter payloads first.
• Add explicit links to the dedicated Search page for query-level features.

Acceptance criteria

• Filters page is clearly scoped to filtering.
• Search-specific features are documented primarily on Search page.
• Cross-links between Filters, Search, and Concepts pages are present and coherent.
• mkdocs build passes.