[Bug]: SolidStart v2 section leaks into Latest sidebar and inline links 404

[tknkaa]

📝 Issue Description

Three related routing bugs affect the SolidStart v2 documentation:

1. A "V2" section appears in the Latest SolidStart sidebar — the start, latest sidebar is built with createFilesystemSidebar("./src/routes/solid-start") and no filter, so it picks up the v2/ subdirectory and renders an unwanted collapsible group.

2. Inline cross-links on v2 pages 404 — all 17 link occurrences across 12 files in src/routes/solid-start/v2/ use the wrong URL pattern /v2/solid-start/... instead of /solid-start/v2/....

3. Wrong sidebar key for start, v2 — vite.config.ts registers the v2 sidebar under "/v2/solid-start" instead of "/solid-start/v2", causing broken prev/next navigation on v2 pages.

📋 On which page(s) it occurs

• https://docs.solidjs.com/solid-start (Latest — "V2" section visible in sidebar)
• https://docs.solidjs.com/solid-start/v2/building-your-application/api-routes (and all other v2 pages — inline links 404)

🤔 Expected Behavior

• The Latest SolidStart sidebar should not contain a "V2" section. Switching to v2 should only be possible via the version selector dropdown.
• Inline links on v2 pages should resolve correctly to /solid-start/v2/....
• Prev/next navigation on v2 pages should resolve correctly to /solid-start/v2/....

👀 Current Behavior

• A "V2" collapsible group appears at the bottom of the Latest SolidStart sidebar.
• Clicking inline links on any v2 page (e.g. "Data fetching", "API routes", `"use server"`) navigates to a 404 URL like /v2/solid-start/building-your-application/data-fetching.
• The prev/next links on v2 pages also point to broken /v2/solid-start/... URLs.