docs(readme): align with v5 SDK surface

[mourraille]

Summary

Aligns the v5 branch README with the actual v5 SDK surface and the published v5.0.0 release notes.

• Fix package name in install command — `pip install brevo` → `pip install brevo-python`. The previous form installs a different (unrelated) package on PyPI.
• Fix Reference link — was pointing to a personal fork (`mourraille/fern-sdk`); switched to the canonical `getbrevo/brevo-python` v5 branch.
• Add an Upgrading from v4.x section reflecting actual v5 breaking changes — filter keyword rename, `create_batch_events` keyword change, Balance/CRM/Webhooks/model changes, shape collapses (`Union[str, List[str]]`, `Dict[str, int]`), and the 21 top-level imports removed. Mirrors the structure used on `brevo-node@v6` (#88) and `brevo-php@v5` (#131).
• Update v1.x migration callout — recommendation now points to v5.x and the comparison table is refreshed.
• Add a Logging section under Advanced — documents the native `logging` constructor option (`LogConfig` dict or pre-built `Logger`), the built-in `ConsoleLogger`, and the `ILogger` protocol for custom loggers. Includes the silent-by-default note so readers don't think their config is broken.

Test plan

• Render the README on the v5 branch and confirm formatting and the banner image.
• Verify code snippets compile against the v5 SDK surface (constructor signature, `logging` option name, `brevo.core.logging` import path).

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