Document Elixir attachment support

### Core or SDK?

Platform/SDK

### Which part? Which one?

Elixir

### Description

The SDK seems to support attachments, but I can not find anything on it in docs:
https://github.com/getsentry/sentry-elixir/pull/673

See Python as a comparison for the page's location:
https://docs.sentry.io/platforms/python/enriching-events/attachments/
