Enable Prism highlighting and fix code-block display

[sboldyreva]

The @vuepress/plugin-prismjs package shipped in package.json but was never registered, so fences with a language hint (bash, json,

:not(.language-text) > code rule that painted any non-text block's text
in dark blue (#1f3f5b) and a code-fade-mask gradient that assumed a dark
background variable nobody set, this rendered as dark-blue-on-black
strips on the right side of every highlighted block.

Changes:

* Register prismjsPlugin in config-user/plugins.ts so language hints are
  tokenised at build time.
* Scope the inline-code rule to :not(pre):not(.language-text) > code so
  the global #1f3f5b color and light-gray background only apply to
  inline <code>, not to <pre><code> inside .language-* blocks.
* Set --code-bg-color: #2d2d2d (the project's original dark surface)
  and apply it to every language-* wrapper, with #ccc as the default
  text/punctuation color, matching the pre-Prism appearance.
* Add a One-Dark-inspired Prism token palette scoped to
  div[class*="language-"] so bash/json/yaml/php blocks render with
  readable colors on the dark background.
* Unify font-size: 12px across every language-* block so highlighted
  blocks no longer render larger than plain `text` blocks.
* Re-tint the code-fade-mask gradient to the same #2d2d2d background so
  the right-edge fade blends with the block.
* Scope the ELSSteps :deep(code) override to :not(pre) > code so block
  code inside steps uses the unified 12px size while inline code keeps
  the slightly smaller 0.85rem.
* Indent the bullet sub-items under steps 1 and 2 in the AlmaLinux
  enterprise-support doc so they render inside the numbered list
  instead of after it.