Fix the ghost emojis on the website (#5824)

Replaces all ghost icons with an svg of the android 8 ghost emoji, which is the one we
started with. Pulled the svg from
https://github.com/googlefonts/noto-emoji/blob/f931bea0efd67aefdf6beae404e1f3150c90314e/svg/emoji_u1f47b.svg,
but had to fiddle with it a bit because whatever markdown emoji renderer thing didn't
like it at first.

This also makes the emoji available as `:iree-ghost:` in markdown. I wanted to override
just `:ghost:` but you can't add something that's already in the index (see
https://github.com/facelessuser/mkdocs-material-extensions#using-local-custom-icons).

Preview at https://gmngeoffrey.github.io/iree
diff --git a/docs/website/mkdocs.yml b/docs/website/mkdocs.yml
index 70c8e7c..07758e8 100644
--- a/docs/website/mkdocs.yml
+++ b/docs/website/mkdocs.yml
@@ -5,9 +5,8 @@
   name: material
   icon:
     repo: fontawesome/brands/github
-    logo: material/ghost
-  favicon: assets/images/favicon.png
-  logo: assets/images/favicon.png
+    logo: iree/ghost
+  favicon: ghost.svg
   custom_dir: overrides
 
   features:
@@ -80,6 +79,9 @@
   - pymdownx.emoji:
       emoji_index: !!python/name:materialx.emoji.twemoji
       emoji_generator: !!python/name:materialx.emoji.to_svg
+      options:
+        custom_icons:
+          - overrides/.icons
   - pymdownx.tabbed
   - pymdownx.superfences
   - pymdownx.tabbed