Document `LangCacheSemanticCache` in the public docs

[tylerhutcherson]

Summary

LangCacheSemanticCache is part of the public redisvl.extensions.cache.llm API and is covered by tests, but it
is not clearly documented in the public docs beyond the redisvl[langcache] install extra.

Gaps

• No dedicated how-to guide for using LangCache as the LLM cache backend
• Missing from the cache API reference
• Semantic cache docs do not explain when to use LangCacheSemanticCache vs SemanticCache
• Attribute prerequisites and unsupported operations are not called out

Proposed Work

• Add LangCacheSemanticCache to docs/api/cache.rst
• Add a task-oriented guide covering installation, initialization, read-through caching, attributes, TTL, async
  usage, and delete operations
• Link the guide from docs/user_guide/how_to_guides/index.md
• Mention the managed-service option in docs/concepts/extensions.md

Acceptance Criteria

• LangCacheSemanticCache appears in the published API reference
• A user can follow a dedicated guide to configure and use the wrapper
• Docs explain that attributes must already be configured in LangCache
• Docs explain current limitations: no direct vector search, no filter_expression, no update()/aupdate()
• Docs build successfully for the changed pages