fix(docs): render Material icons in grid cards (add pymdownx.emoji)

The "Next steps" cards on the home page showed the raw icon syntax
(`:material-download:{ .lg .middle }`) as text because pymdownx.emoji was not
configured. Add it with Material's twemoji index/generator so `:material-...:`
and `:octicons-...:` render as SVG icons (and the grid cards display correctly).

Author: bilouro

mkdocs.yml

@@ -120,6 +120,9 @@ markdown_extensions:
   - pymdownx.snippets
   - attr_list
   - md_in_html
+  - pymdownx.emoji:
+      emoji_index: !!python/name:material.extensions.emoji.twemoji
+      emoji_generator: !!python/name:material.extensions.emoji.to_svg
   - toc:
       permalink: true