Closes https://github.com/zed-industries/zed/issues/58364

This PR adjusts heading and body text font size (mostly reducing them a
bit), as well as colors (making the body text slightly darker than
headings), and content separation, through adding a bottom border to
headings until H3.

| Before | After |
|--------|--------|
| <img width="2620" height="2044" alt="Screenshot 2026-06-03 at 5 
01@2x"
src="https://github.com/user-attachments/assets/d10c7916-e4cb-4999-8be1-08e26793fd35"
/> | <img width="2622" height="2044" alt="Screenshot 2026-06-03 at 5 
02@2x"
src="https://github.com/user-attachments/assets/13105982-283d-436b-973e-b6198cd2503b"
/> |

Release Notes:

- Improved markdown preview styles, making it more legible and easier to
parse.