Add "Attributes" to Enriching Events

### SDK

Other

### Description

Create a new page that documents "Attributes" under the navigation item "Enriching Event".
See #18729 and #18753

SDKs to update:
- Python 
- Dart and Flutter 

> [!CAUTION]
> This must only be merged once streamed span docs are published 

### Suggested Solution

_No response_