docs: update js runtime docs

[abcxff]

Changes

This PR updates the JavaScript runtime documentation to use Deno instead of Node.js:

• Replaced Node.js http module with Deno's native HTTP server API
• Updated environment variable access to use Deno.env.get() instead of process.env
• Changed server shutdown handling to use await server.finished instead of an empty promise
• Added "access": "public" to the example rivet.json configuration
• Updated the API client initialization comment to specify "API service token"

[abcxff]

• docs: update js runtime docs #2362 👈 (View in Graphite)
• main

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[greptile-apps]

PR Summary

This PR updates the JavaScript runtime documentation to reflect the use of Deno's API instead of Node.js, modernizing the code examples for Rivet actors.

• Updated HTTP server example to use Deno.serve instead of Node.js's http module in /site/src/content/docs/javascript-runtime.mdx
• Changed environment variable access from process.env to Deno.env.get for proper Deno compatibility
• Added access: "public" property to the rivet.json configuration example
• Updated API token reference from "API token" to "API service token" for clarity
• Aligned documentation with actual implementation as shown in the example projects' deno.json files

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