gh-79012: Add asyncio chat server HOWTO

[kovan]

Summary

Adds a focused HOWTO for building a TCP chat server with asyncio streams, as suggested by @ZeroIntensity in the review of #144594:

There are some things here that look useful for the docs, but I don't think they need to be put under an "asyncio tutorial" umbrella. Instead, let's just make a HOWTO page for a chat server, since that seems to be the primary focus of all the content here.

The guide progressively builds from an echo server to a full chat server:

• Starting with an echo server — introduces the streams API (start_server, StreamReader/StreamWriter, write/drain, close/wait_closed)
• Building the chat server — adds client tracking, broadcasting, and cleanup
• Extending the chat server — idle timeouts with asyncio.timeout, plus exercises
• Common pitfalls — forgetting to await, blocking the event loop, fire-and-forget tasks

No overlap with the existing Conceptual Overview — this is purely a practical, hands-on HOWTO.

Test plan

• make -C Doc check passes
• make -C Doc html passes (no warnings)
• All cross-references resolve correctly

🤖 Generated with Claude Code

• Issue: Asyncio Tutorial #79012

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📚 Documentation preview 📚: https://cpython-previews--144604.org.readthedocs.build/