Docs, examples, and migration guidance

- **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:** #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".