docs: clarify API key placeholders and safe diagnostic sharing

[JJasonSun]

Summary

Explain how to replace example API keys and remove private data before sharing diagnostic output.

Run an example → use the complete Dashboard key
Share output → remove credentials, phone numbers, and private call text

Refs #344. The call test did not verify real meeting availability, Goal Runs, or webhooks.

Evidence

• Before: The setup text used sample-looking keys and lacked a shared guide for safe diagnostic output.
  After: README and CONTRIBUTING use <YOUR_CALLE_API_KEY> and explain what to remove. A redacted example shows the intended output; it is not an executable request.
• Validation: The SDK 0.7.1 examples completed an authorized test call and retrieved its result. Python validation passed 51 tests, lint/types, builds, and fresh-package installation checks. CI passed.

Merge Danger

Door: two-way — Revert this PR to restore the previous files.

Blast Radius: Docs

No SDK behavior or version change. This adds guidance, not automatic removal of private data. Package-site documentation updates with a later release.