docs: Fix Python SDK credential examples, env default, and stale links

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Why

The Python SDK docs include a credentials example that raises SkyflowError and claim that env defaults to PROD. The SDK has no default and fails when env is omitted. The docs also describe credential precedence wrongly.

Goal

• docs/advanced_initialization.md: wrap the service account as {'credentials_string': ...} so add_skyflow_credentials() accepts it.
• skyvault/README.md, flowvault/README.md, docs/auth_credentials.md: say that passing more than one credential raises SkyflowError. Remove the claim that env defaults to PROD here and in docs/migrate_to_v2.md.
• docs/migrate_to_v2.md: correct the InsertRequest parameter (table). Fix stale links left by the common/skyvault/flowvault split, broken ../README.md links, and the samples README link and run command. Fix a misleading env-var name, a stale version banner, and inconsistent indentation.
• Not in this PR: flowvault/samples/vault_api/insert_records.py also has the wrong comment "PROD (default)". README.rst isn't used as the PyPI long description, and whether to keep it is the team's decision.

Testing

• Every corrected identifier, signature, default, version, and link target was checked against the SDK source, build files, and samples in this repository at the current main commit. Review comments on this PR link each correction to the source line that supports it.
• No SDK code, samples, or build files changed. Builds, tests, and installers were not run.
• Vale prose checks report no new findings in the changed passages.

Trigger Events

• Deep Analysis: Review each doc in each of the SDK repos for accuracy and adherence to best practices for their respective doc types.

[github-actions]

✅ Gitleaks Findings: No secrets detected. Safe to proceed!