feat: Knowledge base enhancements (diataxis)

• Add neoteroi mkdocs plugin for various aesthetic extensions (cards, timeline etc.)
• Add hooks/site_urls.py for site: site-relative linking.
• Add reusable diataxis neoteroi cards & snippets & images for diataxis quadrants
• Add knowledge base page detailing diataxis quadrants in new home/ section page
• Define & integrate reusable admonition snippet & neoteroi card definitions for diataxis ("knowledge quadrants")
• Convert many page markdown links from relative to now-supported site: form
• Alphabetise & neaten extensions in mkdocs.base.yml, add stylesheets for neoteroi cards and diataxis related styling
• Update site page structuring in howtos/ & other diataxis areas so directories reflect navigation structuring for easier management & much smaller doc directory sizes.