Improve TAGS documentation with short examples

The **[TAGS](https://github.com/monapdx/Modular-Markup-Language/blob/main/DOCS/TAGS.md)** list would be easier for new contributors if each major tag had a tiny example.

**Add examples for:**

- argument
- claim
- evidence
- timeline
- event
- comparison
- media
- caption

**Acceptance criteria:**

- Keep examples short.
- Use valid MML syntax.
- Do not add new tags in this issue.