docs: document attaching the VS Code debugger to a Vite/Fresh dev server

[bartlomieju]

Feedback (#2724) reported that there are no instructions for debugging a Deno
Fresh project (which runs Vite) in VS Code. The existing "Using the debugger"
section only covers the generated single-file Deno launch config, which does
not fit a project whose deno task dev starts a long-running Vite server.

This adds a subsection covering the attach workflow: run the dev server under
the Deno inspector (deno run --inspect -A npm:vite, e.g. as a dev:debug
task) and attach VS Code to port 9229.

Verification status:

• Verified headlessly: I scaffolded a Fresh 2 app (deno task dev is vite)
  and confirmed that running it under --inspect brings up both the Vite dev
  server and a reachable Deno inspector on 9229 (valid target in
  /json/list, Debugger listening on ws://127.0.0.1:9229/...). deno fmt,
  the freshness check, and a local build pass, and the --inspect-wait anchor
  resolves.
• Needs interactive confirmation before merge: that breakpoints set in
  server-side Fresh route code actually bind and pause through Vite's transform
  in VS Code. I could not verify GUI breakpoint binding headlessly. Marking this
  PR as draft pending that check. If breakpoints don't bind cleanly, we should
  adjust the recipe (e.g. source-map / outFiles settings) or note the
  limitation.

Closes #2724.

[bartlomieju]

Verified interactively in VS Code: breakpoints in a Fresh route handler do bind and pause on the request. The one wrinkle is that a breakpoint in a route that hasn't been loaded yet shows as "Unbound breakpoint" / "Some of your breakpoints could not be set" until Vite loads that module on first request, after which it binds and pauses normally. Added a note documenting that, and marked the PR ready for review.