Add revealjs ‘clean’ theme

[grantmcdermott]

What does this PR do?

Adds a link to a new "clean" Quarto revealjs theme.

Repo: https://github.com/grantmcdermott/quarto-revealjs-clean

Requirements for your Awesome item list

• Only has awesome items. Awesome lists are curations of the best, not everything.
• Does not contain items that are unmaintained, has archived repo, deprecated, or missing docs. If you really need to include such items, they should be in a separate Markdown file.
• Entries have a description, unless the title is descriptive enough by itself. It rarely is though.
• Has consistent formatting and proper spelling/grammar.
  - The link and description are separated by a dash.
  Example: - [AVA](…) - JavaScript test runner.
  - The description starts with an uppercase character and ends with a period.
  - Consistent and correct naming. For example, Node.js, not NodeJS or node.js.
• Doesn't use hard-wrapping.
• Your item(s) fit into an existing sections. (if you think a new section is needed, please explain why your item does not fit into an existing one and what would be the interest of your new section.
• Add your new item to the bottom of the list in a section.
• If a duplicate item exists, discuss why the new item should replace it.
• Check your spelling & grammar.
• The item must follow this format:

  - [item name](https link) - Description beginning with capital, ending in period.
  

• Be sure to have read the contributing guidelines.

[grantmcdermott]

(As an aside, I just added the theme to the bottom of the list. But I wonder if they should be in alphabetical order for ease of use?)

[mcanouil]

Thanks for the PR.
Awesome list guidelines is by "added date order", so adding it at the bottom was the correct thing to do.

I am also working on something to list extensions, which could be an update of https://quarto.org/docs/extensions/.