Use plausible configuration by the PyData Sphinx Theme (#453)

# References and relevant issues
Similar to napari/island-dispatch#8

# Description
This PR sets up plausible using the [PyData Sphinx Theme configuration
value](https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/analytics.html#plausible-analytics)
instead of using a hand-written template.

Author: melissawm

docs/_templates/layout.html

@@ -105,7 +105,7 @@
         {"name": "Island Dispatch", "url": "https://napari.org/island-dispatch"},
         {"name": "Community chat", "url": "https://napari.zulipchat.com"},
         {"name": "workshop template", "url": "https://napari.org/napari-workshop-template"},
-        ],
+    ],
     "github_url": "https://github.com/napari/napari",
     "navbar_start": ["navbar-logo", "navbar-project"],
     "navbar_end": ["version-switcher", "navbar-icon-links"],
@@ -120,6 +120,12 @@
     "pygment_dark_style": "napari",
     "announcement": "https://napari.org/dev/_static/announcement.html",
     "back_to_top_button": False,
+    "analytics": {
+        # The domain you'd like to use for this analytics instance
+        "plausible_analytics_domain": "napari.org",
+        # The analytics script that is served by Plausible
+        "plausible_analytics_url": "https://plausible.io/js/plausible.js",
+    },
 }
 
 html_context = {