docs: use explicit API key placeholders

[JJasonSun]

Authentication and Quickstart use <YOUR_CALLE_API_KEY> and tell readers to replace it with the complete Dashboard-issued key. The authentication guide retains the documented iams_live_ production-key prefix.

Validation:

• Audited docs source, examples, and generated outputs for ambiguous sample credentials. Existing angle-bracket placeholders and isolated test credentials require no change.
• The authenticated read-only Goals request returned HTTP 200.
• Ran the documented complete Calls example locally with published calle-ai 0.7.0 on Python 3.11.16 and the authorized test hotline: start and resume both exited zero; the call completed with task_completed=true and heard_greeting=yes. Resume reads the saved Call ID and creates no additional call.
• Full validation passed: typecheck, build, distribution checks, 13 deployment tests, and 22 browser tests.
• Inspected Authentication and Quickstart rendering; their Markdown outputs and llms-full.txt contain the explicit placeholder guidance. The existing prefix documentation and changelog remain unchanged.

No authentication implementation, accepted key format, or SDK package changes. No SDK release is required. Production verification is pending merge and deployment.

[JJasonSun]

Documentation follow-up: #63 is merged and deployed. Authentication and Quickstart use an explicit <YOUR_CALLE_API_KEY> placeholder with replacement instructions; the documented production prefix remains unchanged.

Production verification passed: the two pages, their Markdown outputs, and llms-full.txt match the release artifact. Deployment: https://github.com/CALLE-AI/calle-docs/actions/runs/34950896736.