Docs: Add privacy-friendly web analytics with plausible

NOTES_FOR_MAINTAINERS.md

@@ -73,3 +73,7 @@ with the new minimum required version of vl-convert.
 
 To cut a new release of Altair, follow the steps outlined in
 [RELEASING.md](RELEASING.md).
+
+## Web analytics
+We use the privacy-friendly [plausible.io](https://plausible.io/) for tracking usage statistics of our documentation.
+It is hosted on [https://views.scientific-python.org](https://views.scientific-python.org). To view the stats, you need an account. Ask another maintainer to invite you.

doc/conf.py

@@ -159,6 +159,10 @@
     ],
     "header_links_before_dropdown": 6,
     "announcement": """This website is for version 5. You can find the documentation for version 4 <a href='https://altair-viz.github.io/altair-viz-v4/'>here</a>.""",
+    "analytics": {
+        "plausible_analytics_domain": "altair-viz.github.io",
+        "plausible_analytics_url": ("https://views.scientific-python.org/js/script.js"),
+    },
 }
 
 html_context = {"default_mode": "light"}