docs: switch Sphinx theme from furo to sphinx_rtd_theme

Use the canonical Read the Docs theme. Build dropped from 33 warnings
to 3 (most furo warnings were sidebar template lookups in the dark/light
mode switcher).

requirements.txt updated to pull sphinx-rtd-theme instead of furo.

docs/_source/conf.py

@@ -35,7 +35,7 @@ napoleon_numpy_docstring = True
 napoleon_attr_annotations = True
 
 # HTML output
-html_theme = "furo"
+html_theme = "sphinx_rtd_theme"
 html_static_path = ["_static"]
 html_title = "Aborist API Reference"
 

docs/_source/requirements.txt

@@ -1,3 +1,3 @@
 # Sphinx documentation build dependencies for Read the Docs
 sphinx>=8.0
-furo>=2024.0
+sphinx-rtd-theme>=3.0