Document UseDefaultFiles ordering requirement for compressed default documents

[Copilot]

Fixes #36469

The Static files article documents both MapStaticAssets (endpoint-based, serves build-time compressed assets) and UseDefaultFiles (a URL rewriter), but omits the ordering interaction between them: a default document is only served as a compressed static asset when the UseDefaultFiles path rewrite runs before endpoint routing matches the request — i.e., UseDefaultFiles must be called before MapStaticAssets.

Changes to aspnetcore/fundamentals/static-files.md:

• Note near MapStaticAssets compression benefits — Explains that default documents are served compressed only when UseDefaultFiles runs before Routing Middleware, with the why (the / → /index.html rewrite must precede endpoint matching) and the actionable guidance (call UseDefaultFiles before MapStaticAssets). Phrased to avoid implying readers must manually add UseRouting.
• Note in Serve default documents section — Condensed, MapStaticAssets-scoped (.NET 9+) reminder aimed at readers troubleshooting "why aren't my default docs compressed?".
• Metadata — Updated ms.date; added ai-usage: ai-assisted under title.

Both notes are self-contained; the second is intentionally shorter to minimize duplication while remaining discoverable in context.

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aspnetcore/fundamentals/static-files.md	Static files in ASP.NET Core

[wadepickett]

This following note is the main point of this PR as related to the issue it fixes. Everything else changed in the PR is just some clean up per the style guide spotted along the way and fixed.

[wadepickett]

@tdykstra, could use your review.