Add documentation for SCSS variables in revealjs

[cderv]

This is a companion PR to Revealjs callout update to be merged in

• Improve SCSS in Revealjs - Part 2 : Callouts  quarto-cli#11251

It adds documentation for SCSS variable available to customize callouts.

@cwickham this PR also adds a new feature for our doc: Adding a color box next to some part mentioning a color.

Basically doing [`<hexcolor>`]{.color-box} would use the hexcolor to create the box. It would work with any text using [any content]{.color-box color="hexcolor"}

We could probably use that into the website in other places, and we could also make it a shortcode syntax if easier.

I noticed a few changes in preview locally regarding tables in documentation. I'll check if this is happening with the preview here too. I wonder if our brand change have had side effect

[github-actions]

🚀 Deployed on https://deploy-preview-1446.quarto.org

[cderv]

Update is here: https://deploy-preview-1446.quarto.org/docs/presentations/revealjs/themes.html#callouts

I noticed a few changes in preview locally regarding tables in documentation. I'll check if this is happening with the preview here too. I wonder if our brand change have had side effect

We don't see the darker border in table, so this is coming to latest dev version not yet built in a pre-release

[cderv]

@cwickham just asking for review if you think this should be done different, or more explicit. And to have feedback on the filter idea.

Then this is to be merged after quarto-cli PR is merged

[cderv]

Then this is to be merged after quarto-cli PR is merged

PR is merged. Waiting for confirmation here before merging

[cwickham]

This looks great! I love the colorbox idea.