Add docs warning about ProactorEventLoop and async DB drivers on Windows

[baltika10]

Summary

Adds a warning admonition to the Event Loop docs page about ProactorEventLoop being incompatible with async database drivers (psycopg, asyncpg) that rely on add_reader()/add_writer() on Windows.

The warning includes:

• Which drivers are affected and why
• The --loop asyncio:SelectorEventLoop workaround using the existing custom event loop syntax
• The UVICORN_LOOP environment variable as a set-once alternative

Related discussion: #2749

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

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_{Comparing baltika10:docs/windows-proactor-async-db-warning (a760446) with main (6761b2c)}