Study recruitment information for Aachen, Frankfurt and Heidelberg, taken
from the sites' study flyers. Each site is a collapsible panel (native
<details>, no JavaScript) with the study description, inclusion criteria,
what participation involves, compensation, contact details and a download
button for the original PDF.
Content lives entirely in the front matter of content/participate/ under
`locations:`, so adding a site needs no template change.
The main menu gains a "Teilnahme" / "Participate" entry right after News,
rendered as a highlighted button. Congo's header partial has no hook for
styling a single entry, so static/css/header-cta.css matches it by href
instead of forking the 234-line theme template; it is loaded site-wide via
the theme's extend-head.html hook.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Jennifer Krzemien
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