Docs, examples, and migration guidance

[jlrobins]

• Goal: a coherent interactive-OAuth section for the driver's docs.
• Scope (in): README / DBAPI_EXTENSIONS.md OAuth section — one login replaces the
  global/flink/tableflow/connect key matrix and unlocks the CMK lookup; one browser bounce per
  process even under dbt; the borrowed-client_id port-collision caveat; CHANGELOG.md; an
  examples/ interactive-login script.
• Note: per-feature docs land with their feature (child 5); this child is the consolidation
  pass + the worked example, not the only place docs appear.
• Depends on: connect() wiring for interactive OAuth #155
• Prior art: mcp-confluent is the template to parallel — README.md "OAuth Authentication for
  Confluent Cloud", CONFIGURATION.md "OAuth connection (type: oauth)", a config.oauth.example.yaml
  starter, and a runnable sample config. ide-sidecar has no dedicated OAuth doc — only
  CONTRIBUTING.md's "Create a new CCloud connection and sign in".