@theme-ui/prism README improvements

[deepsheth]

Is your feature request related to a problem? Please describe.
On the @theme-ui/prism docs, the Highlight section is a little hard to follow considering:

1. the code block adds the // highlight-line tag in the code, which is then trimmed by the ThemeUIPrism component, but the background is not highlighted in the docs. Likewise for the mulit-line highlight (see the first screenshot).
2. The presets directory has more themes than what the docs list. Maybe that should be pointed out or the list should be updated?

Describe the solution you'd like

• For the Highlight docs, I think it's easier to read the prism/README.md on Github which preserves the highlight- comments
• Since the .highlighted class is added to highlighted lines, we could add that class to the doc's theme file to match what the docs say:

The highlight styling is controlled in styles.pre under .highlight. For example:
".highlight": {
"background": "hsla(0, 0%, 30%, .5)"
}

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context

Current
Styling .highlight class with a background
GitHub docs