Support email file formats: `.eml` and `.msg`

Ability to parse `.eml` and `.msg` files would be great.

`docling` [supports](https://docling-project.github.io/docling/usage/supported_formats/#supported-input-formats) this:

> _MIME (.eml) and Outlook (.msg) email messages; attachment names can optionally be listed via EmailBackendOptions_

No implementation files or tests are named. Review the repository's existing format-support entry points and the linked Docling supported-formats reference first, then trace how current document inputs are parsed. Done means both .eml and .msg files can be converted successfully, with attachment-name handling clarified and covered by tests.

Written by the indexing model from the issue text.