Replace hardcoded API key in docs with env var or secrets example

[rcsmit]

The documentation and examples show an API key inline, like:
generation_api_key="your-api-key"

Even as a placeholder, this pattern encourages people to paste real keys into code, which often ends up in git history, screenshots, logs, or shared snippets.

Why this matters

• Hardcoded keys get leaked easily
• Leaked keys can be abused and create unexpected costs
• Many users copy examples verbatim

Proposed change
Update all examples to use environment variables or a secrets mechanism.

Suggested examples
Python

import os
generation_api_key = os.getenv("GENERATION_API_KEY")

Acceptance criteria

• No docs show API keys inline, even as placeholders
• Docs include a short “Set your key via env var” step
• Optional: add a short note “Never commit your key” near the snippet