docs: fix README validate order and plain-language pass

[lesnik512]

Summary

README fixes from a docs review.

The usage example called container.validate() before DIInterceptor(container). The interceptor constructor registers grpc_context_provider (modern_di_grpc/main.py, DIInterceptor.__init__ -> _ensure_context_provider), so any provider depending on grpc.ServicerContext fails validation if validate() runs first. Reproduced with a minimal script: validate() before the interceptor raises ValidationFailedError; after it, it passes.

Changes

• Move container.validate() after the server/interceptor is built, with the same comment the other integration READMEs use
• Rewrite the async/lifecycle paragraph without version history ("As of modern-di 3.1", "no longer need to") or bold phrases
• Replace the em dash in the Usage intro
• Shared footer: "with an IoC container"; org link sentence without a dash

examples/app.py had the same order and now validates after constructing the interceptor.

Checklist

• Lint and format pass (ruff)
• Type check passes (ty)
• Tests pass and new behavior is covered
• Build succeeds (uv build) if packaging or build config changed
• Repo metadata stays consistent across the three surfaces (GitHub description, pyproject description, profile blurb) if this touches packaging