docs(v2/httpserver): add README

Elliot Forbesrequested to merge
ef-v2-httpserver-readme into master

What

Adds a README.md for the v2/httpserver package.

Why

httpserver is the package most new service authors will reach for first, but it was the only major v2 package without a README. The doc.go is comprehensive but not rendered in GitLab's file browser or most editors.

What's covered

  • Quick start showing wiring with app.App
  • Built-in middleware table (tracing + logging)
  • Routing patterns: path params, Group, Route, Mount
  • Health endpoints (/-/liveness, /-/readiness) with readiness check examples and sample 503 response body
  • Full configuration reference with defaults
  • Testing patterns: ServeHTTP without a live port, and :0 address for integration tests

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