feat: Knowledge base enhancements (diataxis)

See merge request !453

chore: Temporarily add test links to deduce site: link format compatible with Gitlab artifact site browsing

fix: Update mkdocs config to lookup SITE_URL in test CI job otherwise use existing predefined Guidebook URL

fix: Convert Diataxis card definitions to use <img> instead of plugin-provided image facility, so 'site:' path can be used, to support card presentation at different depths in page/docs hierarchy