Update Perplexity documentation

[kesku]

Documentation updates for langchain-perplexity to align with the SDK overhaul introduced in langchain-ai/langchain#34412.

Type of change

Type: Update existing documentation

Related issues/PRs

• Feature PR: feat(perplexity): overhaul integration with official SDK and Search API langchain#34412

Checklist

• I have read the [contributing guidelines](README.md)
• I have tested my changes locally using docs dev
• All code examples have been tested and work correctly
• I have used root relative paths for internal links
• I have updated navigation in src/docs.json if needed

Additional notes

Changes:

• Added new documentation pages for PerplexitySearchRetriever
• Added new documentation pages for PerplexitySearchResults
• Expanded ChatPerplexity documentation:
  • Updated feature support table (Serializable, native async, image input)
  • Added Pro Search, Search Controls, and Async usage sections
  • Added image input model compatibility note
• Fixed PerplexitySearchResults instantiation example (removed invalid parameters)
• Added advanced usage examples for PerplexitySearchResults
• Updated providers page to link to new retriever and tool documentation
• Fixed model cards documentation link
• Updated agent example to use create_react_agent