docs: move MCP section to top and clarify langchain-mcp-adapters usage

[Nomzy-kush]

Added section on Model Context Protocol (MCP) support and its benefits.

Overview

This PR makes the langchain-mcp-adapters package easier to find. It adds a new “Model Context Protocol (MCP)” section at the top of the Integrations Overview page.
MCP support was only previously mentioned in a tip block. This made it hard for developers to find when looking for integration options. This update shows that MCP is an important integration. It also explains its purpose and value for users of LangChain and LangGraph.

Type of change

Type: Update existing documentation

Related issues/PRs

Fixes #32395

Checklist

• I have read the contributing guidelines
• 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
• I have gotten approval from the relevant reviewers
• (Internal team members only / optional) I have created a preview deployment using the Create Preview Branch workflow