Documentation uses reexports that are not included in `__all__`

**Describe the bug**:
The documentation for this library references decorators like `@elasticapm.capture_span()` or functions (e.g. `elasticapm.instrument()`) however they are not included in the `__all__` variable. This causes type checkers (e.g. pyright/mypy) to complain because they expect explicit reexports via `__all__`. What's weird is that `__all__` is defined, but the intended reexports (at least from looking at the documentation) are not added.

This makes it extremely confusing for someone that is new elasticapm, and the code becomes littered with`# pyright: ignore` comments or imports from various subpackages.

**To Reproduce**

1. Install pyright
2. Follow documentation (e.g. use a capture_span like so https://www.elastic.co/guide/en/apm/agent/python/current/instrumenting-custom-code.html#instrumenting-custom-code-spans)
3. Verify that pyright will complain that `"capture_span" is not exported from module "elasticapm" (Pyright reportPrivateImportUsage)`

**Environment (please complete the following information)**
- Agent version: 6.23.0