Improve custom listings documentation to avoid caveats

[mcanouil]

What would you like to do?

Give feedback or suggest an improvement

Description

As (re)discovered recently (#11760), EJS templates are in fact read as Markdown in Quarto, this means for safety but not only (#11701), HTML code should be enclosed in raw content blocks, except if some elements are to be evaluated as Markdown such as title, description, etc.

The current state of the documentation implies EJS files provided, as custom listings template, are pure HTML.

https://quarto.org/docs/websites/website-listings-custom.html

More advanced elements could be added such as the difference between the use of inline versus raw content block in the AST, such that Markdown lines are enclosed in <p> tags.