Document DENO_ Prefix Restriction for Environment Variables in Deploy

[abhi-parate-st3]

Currently, there's a disconnect between Deno runtime and Deno Deploy documentation regarding environment variables with the DENO_ prefix:

1. The runtime documentation lists several special environment variables starting with DENO_ that developers can use during development.
2. However, when deploying to Deno Deploy, setting any environment variable with the DENO_ prefix results in an error:
   Invalid key: DENO_ is a reserved prefix
3. This restriction is not documented in the Deno Deploy environment variables documentation.

Impact:

• Developers following the runtime docs may use DENO_ prefixed variables during local development
• These applications will fail when deployed due to the undocumented restriction
• This creates a frustrating developer experience and requires unnecessary debugging time

Proposed Documentation Update:
Add a prominent note in the Deploy environment variables documentation stating:

Important

Environment variables with the DENO_ prefix are reserved and cannot be set in Deno Deploy. While these variables (like DENO_AUTH_TOKENS) work in local development, you'll need to use alternative approaches when deploying.

Additional Suggestions:

1. List common DENO_ variables that developers might try to use and their alternatives in Deploy
2. Consider adding this warning to the runtime docs where DENO_ variables are mentioned

This documentation update will help developers avoid deployment issues and provide a smoother development experience.