docs(mkdocs): drop fontawesome_markdown + mermaid extras

The new lens image doesn't include `fontawesome_markdown` (and hologit
dropped it from their own lens config), so mkdocs build aborts with
ModuleNotFoundError. Nothing in `docs/` uses `:fa-icon:` syntax, so
the extension is safe to remove outright.

While in there:

- dropped the FontAwesome v5 CDN CSS link (Material for MkDocs already
  ships an SVG icon set — `:material-…:` / `:fontawesome-…:` syntax —
  via `pymdownx.emoji` if we ever want icons again)
- dropped the Mermaid v8 CDN extra_javascript (none of the docs use
  Mermaid diagrams; if we want them, Material's `pymdownx.superfences`
  with the mermaid fence format is the modern path)
- simplified `pymdownx.superfences` to its default config (the custom
  Mermaid fence is gone with the JS)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: themightychris

mkdocs.yml

@@ -37,12 +37,6 @@ nav:
 plugins:
   - search
 
-extra_javascript:
-  - https://unpkg.com/mermaid@8.5.0/dist/mermaid.min.js
-
-extra_css:
-  - https://use.fontawesome.com/releases/v5.13.0/css/all.css
-
 markdown_extensions:
   - admonition
   - codehilite:
@@ -51,16 +45,10 @@ markdown_extensions:
   - pymdownx.tasklist:
       custom_checkbox: true
   - pymdownx.tabbed
-  - pymdownx.superfences:
-      custom_fences:
-        - name: mermaid
-          class: mermaid
-          format: '!!python/name:pymdownx.superfences.fence_div_format'
+  - pymdownx.superfences
   - pymdownx.smartsymbols
   - meta
   - toc:
-      # insert a blank space before the character
       permalink: " ¶"
   - smarty
-  - fontawesome_markdown
   - mdx_truly_sane_lists