docs: serve Google Search Console verification file via html_extra_path

Keeps googlecbf55b468ef76c2e.html in the published site root so it survives
every docs rebuild (publish-docs.yml deploys with rsync --delete).

Author: avanturist322

docs/source/_extra/googlecbf55b468ef76c2e.html

@@ -0,0 +1 @@
+google-site-verification: googlecbf55b468ef76c2e.html

docs/source/conf.py

@@ -40,6 +40,8 @@
 templates_path = ["_templates"]
 exclude_patterns = ["_build", "Thumbs.db", ".DS_Store"]
 html_static_path = ["_static"]
+# Files copied verbatim to the site root (e.g. Google Search Console verification).
+html_extra_path = ["_extra"]
 
 html_theme = "pydata_sphinx_theme"
 html_theme_options = {