fix(docs): pin pygments to 2.19.2 and keep syntax highlighting

[lafirm]

Description

Closes #5853

Fixes the docs build failure by pinning pygments to 2.19.2 (prior to buggy version 2.20.0) in the docs dependency set, instead of disabling pygments globally (by updating mkdocs.yml)

# alternate fix by disabling pygments
  - pymdownx.highlight:
      anchor_linenums: true
      use_pygments: false

Problem

• Builds started failing with a pymdownx highlight to pygments HTML formatter error after dependency resolution moved to pygments 2.20.0.
• Pinning to 2.19.2 restores stable builds while preserving code-block highlighting and related formatting behaviour.
• This is a lower-impact fix than turning highlighting off globally.

What changed

• Pinned pygments to 2.19.2 (below 2.20.0) in docs dependencies to keep syntax-highlighting behaviour enabled

Related Links

• Pygments 2.20.0 section: https://pygments.org/docs/changelog/#version-2-20-0
• Related issue: Superfence warning with pygments 2.20.0 pygments/pygments#3077 (comment)

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO