feat(docs): Add Plausible analytics to Sphinx docs

Sphinx docs had no usage analytics, so there was no visibility into
which API reference pages get traffic. Overrides the shibuya theme's
empty extra-head partial to load the Plausible script.

Fixes PY-2757
Fixes #7318

Author: ericapisani

docs/_templates/partials/extra-head.html

@@ -0,0 +1 @@
+<script defer data-domain="getsentry.github.io/sentry-python" src="https://plausible.io/js/script.js"></script>