docs(site): add FLATFILES section to the docs-site

[userFRM]

Problem

The VitePress sidebar at docs-site/docs/.vitepress/config.ts has top-level entries for Historical Data and Real-Time Streaming only — no Flat Files. docs-site/docs/index.md line 7 tagline says "Historical (MDDS gRPC) and real-time (FPSS) surfaces." docs-site/docs/api-reference.md line 8 prose echoes the same two-surface story.

Proposed solution

• Add docs-site/docs/flatfiles/index.md (overview + SecType × ReqType matrix), connecting.md (auth + endpoint), request.md, formats.md (CSV vs JSONL vs raw blob), decoded-rows.md, tier-gating.md (option-flatfile bundle vs the still-blocked stock-flatfile bundle per ROADMAP.md).
• Add a Flat Files block to the sidebar between Real-Time Streaming and More.
• Update index.md tagline and feature cards to list three surfaces.
• Update api-reference.md line 8 and add a ## Flat Files section after the existing ## Streaming (FPSS) section, with code samples for Rust / Python / TypeScript / Go / C++ once the binding issues land.
• Cross-link to vendor docs https://http-docs.thetadata.us/operations/get-v2-flat-file-getting-started.html.

Impact

Docs-site visitors see the third surface. Same depth of coverage as MDDS and FPSS.

[userFRM]

Closed by PR #514. docs-site has a dedicated FLATFILES section (commit 00acee4).