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

chore: Updated many pages to use 'site:' links instead of relative links, for pages that have moved in this change

chore: Update site page structuring in howtos & other diataxis areas so directories reflect navigation structuring for easier management & much smaller doc directory sizes

feat: Alphabetise & neaten extensions in mkdocs.base.yml, add stylesheets for neoteroi cards and diataxis related styling