Move AsyncMinHashLSH from experimental to datasketch.aio

[Copilot]

What does this PR do?

Promotes AsyncMinHashLSH from experimental to stable API with new import path datasketch.aio. Old imports remain functional with deprecation warnings.

New structure:

• datasketch/aio/ - New submodule containing AsyncMinHashLSH, AsyncMinHashLSHInsertionSession, AsyncMinHashLSHDeleteSession
• datasketch/__init__.py - Optional export (gracefully handles missing motor/redis dependencies)
• datasketch/experimental/aio/ - Deprecated shim with warnings, re-exports from new location
• pyproject.toml - Renamed dependency group experimental_aio → aio (kept alias)

Import paths:

# New (recommended)
from datasketch.aio import AsyncMinHashLSH
from datasketch import AsyncMinHashLSH  # requires motor or redis.asyncio

# Old (deprecated, emits DeprecationWarning)
from datasketch.experimental.aio import AsyncMinHashLSH

Changes:

• Updated all documentation references (docs/lsh.rst, docs/documentation.rst) to use datasketch.aio
• Removed "experimental" designation from async LSH documentation
• Fixed RedisStorage import to be conditional (optional dependency handling)
• Updated tests to use new import path
• Added clarifying comments for optional dependency patterns

Checklist

• Are unit tests passing?
• Documentation added/updated for all public APIs?
• Is this a breaking change? If yes, add "[BREAKING]" to the PR title.

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