docs: add fastify middleware to top-level README listing

[Joosboy]

Summary

Adds the @modelcontextprotocol/fastify package to the main repository README.md in three places (Overview, Middleware packages listing, and Installation examples) alongside the other middleware integrations.

Closes #2112.

Test Plan

• Inspected README.md formatting visually.
• Verified that all installation snippets and references are correct and align with the other middleware packages.

[changeset-bot]

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Latest commit: 360151b

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@modelcontextprotocol/client

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@modelcontextprotocol/codemod

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@modelcontextprotocol/core

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@modelcontextprotocol/server

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@modelcontextprotocol/server-legacy

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@modelcontextprotocol/express

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@modelcontextprotocol/fastify

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@modelcontextprotocol/hono

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@modelcontextprotocol/node

npm i https://pkg.pr.new/@modelcontextprotocol/node@2497

commit: 360151b