migrate to decoupled Blowfish layout, fix TOC scrollspy, and update docs

- Theme Decoupling & Cleanup:
      - Fully decoupled and vendored the Blowfish theme directly into root directories (layouts/,
  assets/, static/, data/, i18n/).
      - De-registered and cleaned up legacy Git submodules (themes/blowfish, themes/hugo-admonitions)
  from the Git index to protect the build from upstream breakages.
      - Configured .gitignore to ignore development preview symlinks (content, static,
  config/_default/menus.en.toml), build folders (public/, resources/), and locks.

    - TOC & Layout Fixes:
      - Replaced the custom layouts/_default/single.html template with the Blowfish default to restore
  native container widths and the side-by-side flex layout.
      - Restored TOC to float on the right sidebar on desktop viewports and render inline on mobile
  viewports.
      - Fixed a parameter resolution bug in layouts/partials/toc.html by checking both .Site.Params.
  smartTOC and .Site.Params.article.smartTOC (where it is defined in params.toml).
      - Upgraded the TOC scrollspy JavaScript to target all '#TableOfContents' containers on the page,
  enabling highlight tracking on both desktop and mobile layouts.

    - Documentation Updates (README.md):
      - Restored YAML front matter metadata block at the top.
      - Added detailed notes on the CI/CD build process and Gitea runner deployment targets
  (/srv/www/docs-public and /srv/www/docs-private).
      - Added a categorized and sorted "Shortcodes Quick Reference" table at the top of the shortcodes
  section (ordered by Custom, Hugo Default, and Blowfish).
      - Documented custom shortcodes (Include, Screenshot, Raw HTML) and missing Hugo default
  shortcodes (Highlight, Instagram, Param, Ref, Relref, Twitter, Vimeo, Youtube).
      - Restructured headings to ensure all sub-shortcodes are H3, removed block break tags (<br/>),
  and appended horizontal separators (---) after H2 sections.

assets/icons/xing.svg

@@ -0,0 +1 @@
+<svg role="img" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg"><title>Xing</title><path fill="currentColor" d="M18.188 0c-.517 0-.741.325-.927.66 0 0-7.455 13.224-7.702 13.657.015.024 4.919 9.023 4.919 9.023.17.308.436.66.967.66h3.454c.211 0 .375-.078.463-.22.089-.151.089-.346-.009-.536l-4.879-8.916c-.004-.006-.004-.016 0-.022L22.139.756c.095-.191.097-.387.006-.535C22.056.078 21.894 0 21.686 0h-3.498zM3.648 4.74c-.211 0-.385.074-.473.216-.09.149-.078.339.02.531l2.34 4.05c.004.01.004.016 0 .021L1.86 16.051c-.099.188-.093.381 0 .529.085.142.239.234.45.234h3.461c.518 0 .766-.348.945-.667l3.734-6.609-2.378-4.155c-.172-.315-.434-.659-.962-.659H3.648v.016z"/></svg>