docs: replace 11ty shortcodes with Nuxt syntax - #8694

@dimitrieh you're working on the docs, not sure if this will clash with your work, but we need to render the form as soon as possible.

Yndira, no clash with my docs work: none of my open PRs touch these files. Looking good from my end!

The form is missing on the live page because the website's docs import silently strips any leftover Nunjucks tag (convertCallouts). This brings it back once merged, since the merge triggers a website rebuild. Two things to know:

• The callouts render as Nuxt UI notes (icon, no "Note"/"Warning" title), the same as the handbook, rather than the current ff-callout boxes. Worth a look on the site once it's live.
• After this, that conversion and the ff-callout styles are unused for the docs. Ideally the website build fails on a leftover {% rather than dropping it, so a missing form like this one gets caught.

Follow-up on the website side: FlowFuse/website#5890

Correction to my comment above: the ff-callout styles were not unused, the website's own pages under /docs still had them. That PR moves those to ::note too, so /docs has one callout style, and a leftover {% now fails the build.