new: add community-maintained extension package explanation

[Myxogastria0808]

The author should do the following, if applicable

This PR adds a short section to the @hono/zod-openapi README introducing a community-maintained extension, zod-openapi-share, which helps centralize and reuse common OpenAPI response definitions.

The extension is clearly marked as non-official and is intended to complement @hono/zod-openapi in real-world API development.

• Add tests
• Run tests
• yarn changeset at the top of this repo and push the changeset
• Follow the contribution guide

[changeset-bot]

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Latest commit: a3dfcc0

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[Copilot]

Pull request overview

Adds documentation to the @hono/zod-openapi package README to point users to a community-maintained extension for sharing/reusing OpenAPI response definitions.

Changes:

• Introduces a new “Community Extensions” section in the @hono/zod-openapi README
• Adds a short description and links for the zod-openapi-share community package

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