Use GitHub style emojis

The default EmojiOne emoji index is no longer recommended due to licensing issues. Use (GitHub) geomji index and render them as Unicode (default was png).

Only place where we seem to be using emoji so far are the on https://atlas-software.docs.cern.ch/athena/configuration/.

mkdocs.yml

@@ -197,7 +197,9 @@ markdown_extensions:
   - admonition # callouts like !!!note and !!!tip
   - pymdownx.details
   - pymdownx.superfences # code inside admonitions
-  - pymdownx.emoji
+  - pymdownx.emoji:
+      emoji_index: !!python/name:pymdownx.emoji.gemoji
+      emoji_generator: !!python/name:pymdownx.emoji.to_alt
   - pymdownx.snippets:
       url_download: true
       check_paths: true