Fix content tabs on website. (#7805)

See https://squidfunk.github.io/mkdocs-material/reference/content-tabs/

(Broken since around October?)

docs/website/mkdocs.yml

diff --git a/docs/website/mkdocs.yml b/docs/website/mkdocs.yml
index 2d9be54..1178135 100644
--- a/docs/website/mkdocs.yml
+++ b/docs/website/mkdocs.yml
@@ -80,9 +80,9 @@
       options:
         custom_icons:
           - overrides/.icons
-  - pymdownx.tabbed
   - pymdownx.superfences
-  - pymdownx.tabbed
+  - pymdownx.tabbed:
+      alternate_style: true
   - pymdownx.tasklist:
       custom_checkbox: true
   - tables