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

先使用 pyright 复现文档中的 capture_span 示例，然后检查 elasticapm 的 __all__ 定义，并将其与文档中的顶层装饰器和函数进行比较。当文档中的导入被显式导出，并且 pyright 不再报告对私有导入的使用时，即表示完成。

由索引模型根据 Issue 内容生成。