feat: news media offset via core-styles mixin (#1059)

## Overview

Offset news media —

* main image (atop content)
* secondary images, _et cetera_ (in content)

— via Core-Styles mixin `offset-content`.

## Related

- [RT
#37827](https://tickets.tacc.utexas.edu/Ticket/Display.html?id=37827)
- requires TACC/Core-Styles#577

## Changes

- **changed** raw style to mixin for content media offset
- **added** offset mixins that themselves use Core-Styles offset mixin
- **changed** raw style to mixin for main media offset

## Testing

0. Have:
    - news article with image
    - thumbnail option of full-width i.e. Width: 0
    - thumbnail option of set width i.e. Width: 300
2. Set main image to thumbnail option full-width.
    - On wide screen, image is as wide as content.
    - On narrow screen, image is as wide as content.
2. Set main image to thumbnail option set width.
    - On wide screen, image is offset to the right.
    - On narrow screen, image is floated to the right.

## UI

https://github.com/user-attachments/assets/e061e967-f6b1-4efa-b439-f2efcae49fde

https://github.com/user-attachments/assets/8f819ce9-2b7f-409b-8394-1f32d1f5835e

Author: wesleyboar