API docs

[nstrayer]

This PR substantially increases the scope of the existing extension-development page and adds a new one called api-advanced that goes into further depth regarding using the Positron API in extensions.

Docs now reference the new @posit-dev/positron NPM package and a template repo for getting started with an extension.

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[juliasilge]

Thank you so much for working on this! And for all the great work on the npm package; this is going to improve the situation for extension developers so much 🙌

[Copilot]

Pull Request Overview

This PR updates the extension development documentation to expand the guidance on using the Positron API, and to improve the setup and example sections for building extensions. Key changes include:

• Renaming and reorganizing sections (e.g., "Context keys" to "Prerequisites") for better clarity.
• Adding detailed setup instructions and code examples for activating Positron features.
• Updating guidance on Positron-specific API usage and versioning.

[juliasilge]

Thank you so much!