Add npm packages readmes

[petyosi]

No description provided.

[Copilot]

Pull Request Overview

This PR adds comprehensive README files for three npm packages in the Logfire JavaScript SDK ecosystem. The purpose is to provide proper documentation for each package that will be displayed on npm when users view the packages.

• Replaces placeholder README content with detailed package descriptions
• Adds usage examples and cross-references between related packages
• Standardizes documentation structure across all three packages

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
packages/logfire/README.md	Adds comprehensive Node.js SDK documentation with installation and usage example
packages/logfire-cf-workers/README.md	Adds Cloudflare Workers SDK documentation with cross-references to other packages
packages/logfire-browser/README.md	Creates new browser SDK documentation with usage guidance and example links

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