Refresh website branding. (#16151)

Context: https://groups.google.com/g/iree-discuss/c/kdFr4CQwEyY

This is mostly a revert of https://github.com/openxla/iree/pull/12229,
moving back to mostly default colors and IREE icons. We can add back
custom colors etc. later (e.g.
https://squidfunk.github.io/mkdocs-material/setup/changing-the-colors/).

Current site: https://iree.dev/
Preview: https://scotttodd.github.io/iree/
diff --git a/docs/website/mkdocs.yml b/docs/website/mkdocs.yml
index d477146..071533c 100644
--- a/docs/website/mkdocs.yml
+++ b/docs/website/mkdocs.yml
@@ -6,7 +6,7 @@
 
 theme:
   name: material
-  logo: assets/images/openxla-logo-lockup-white.svg
+  logo: assets/images/ghost.svg
   logo_alt: IREE
   icon:
     repo: fontawesome/brands/github
@@ -14,9 +14,8 @@
   font:
     text: Noto
     code: Noto Sans Mono
-  favicon: assets/images/favicon.png
+  favicon: assets/images/ghost.svg
 
-  # Note: this includes symlinks to forked files in third_party/mkdocs-material/
   custom_dir: overrides
 
   features:
@@ -37,13 +36,13 @@
     - toc.follow # Scroll the TOC panel to follow the reader
 
   palette:
-    # Light mode (scheme overrides in openxla.css)
+    # Light mode
     - media: "(prefers-color-scheme: light)"
       scheme: default
       toggle:
         icon: material/brightness-7
         name: Switch to dark mode
-    # Dark mode (scheme overrides in openxla.css)
+    # Dark mode
     - media: "(prefers-color-scheme: dark)"
       scheme: slate
       toggle:
@@ -51,10 +50,6 @@
         name: Switch to light mode
 
 extra:
-  # TODO(scotttodd): https://openxla.github.io/community (when hosted)
-  community_homepage: https://github.com/openxla/community
-  community_site_name: OpenXLA
-
   homepage: https://iree.dev/
 
   # TODO(scotttodd): join mkdocs-material insiders and remove notice?
@@ -77,7 +72,7 @@
 
 extra_css:
   - assets/stylesheets/extra.css
-  - assets/stylesheets/openxla.css
+  - assets/stylesheets/iree.css
 
 copyright: Copyright © 2024 The IREE Authors
 
@@ -114,7 +109,7 @@
       custom_checkbox: true
   - tables
   - toc:  # Table of Contents
-      permalink: 'link'  # Use Material font's "link" icon; see openxla.css
+      permalink: 'link'  # Use Material font's "link" icon; see iree.css
       toc_depth: 4
 
 # Navigation with explicit ordering and nesting.