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.

layouts/partials/header/fixed-gradient.html

@@ -0,0 +1,18 @@
+<div class="min-h-[148px]"></div>
+<div
+  class="fixed inset-x-0 min-h-[130px] opacity-65 bg-gradient-to-b from-neutral from-60% dark:from-neutral-800 to-transparent mix-blend-normal z-80"></div>
+<div class="fixed inset-x-0 z-100">
+  <div
+    id="menu-blur"
+    class="absolute opacity-0 inset-x-0 top-0 h-full single_hero_background nozoom backdrop-blur-2xl shadow-2xl"></div>
+  <div class="relative m-auto leading-7 max-w-7xl px-6 sm:px-14 md:px-24 lg:px-32">
+    {{ partial "header/basic.html" . }}
+  </div>
+</div>
+{{ $backgroundBlur := resources.Get "js/background-blur.js" }}
+{{ $backgroundBlur = $backgroundBlur | resources.Minify | resources.Fingerprint (.Site.Params.fingerprintAlgorithm | default "sha512") }}
+<script
+  type="text/javascript"
+  src="{{ $backgroundBlur.RelPermalink }}"
+  integrity="{{ $backgroundBlur.Data.Integrity }}"
+  data-blur-id="menu-blur"></script>