Minor branding hints

[alex-r-bigelow]

A few minor improvements from a new-ish / casual Quarto user, who got lost in the docs this past week because the HTML Theming page is the first hit for many Google searches

Fixes:

• The HTML Theming page has no direct links to _brand.yml; this should probably be the first thing a new / casual user tries, before (html-only) "Basic Options?"
• "Basic Options" is written mostly from the perspective of per-document changes, which is a little odd / confusing on a "Theming" page, especially as the rest of the SASS theme variables on the page seem to be describing more site-wide theming
• For someone out of the loop, it's easy to miss that _brand.yml is a separate project from Quarto, and all the "Limitations" and references to "the spec" are really confusing. Maybe this is a stretch (feel free to reject / critique this change), but it might have helped me to put the Brand.YML logo at the top of its docs page?

[cscheid]

For someone out of the loop, it's easy to miss that _brand.yml is a separate project from Quarto, and all the "Limitations" and references to "the spec" are really confusing.

Maybe this is a stretch (feel free to reject / critique this change), but it might have helped me to put the Brand.YML logo at the top of its docs page?

I think it's the correct diagnosis, but maybe not the best fix. We should instead spell out more explicitly what you said in the first paragraph of the overview.

[cscheid]

(I'll go over these with @cwickham and get back to you. It might be a week or so, but we appreciate the PR and feedback!)