docs: switch to sphinx-book-theme for unified right-side project TOC

sphinx_rtd_theme only renders left-side navigation. Switch to
sphinx-book-theme which puts the full project TOC on the right and
leaves the reading area centered.

conf.py:
- html_theme = 'sphinx_book_theme'
- html_sidebars = {'**': []} hides the left sidebar so the right TOC
  is the single navigation surface
- show_toc_level=3 expands subpages; show_navbar_depth=2 controls top nav

requirements.txt: sphinx-rtd-theme → sphinx-book-theme>=1.1

docs/_source/conf.py

@@ -35,10 +35,23 @@ napoleon_numpy_docstring = True
 napoleon_attr_annotations = True
 
 # HTML output
-html_theme = "sphinx_rtd_theme"
+html_theme = "sphinx_book_theme"
 html_static_path = ["_static"]
 html_title = "Aborist API Reference"
 
+# Move the project TOC to the right side. Reading area stays centered;
+# left-side primary sidebar is hidden so the right TOC is the single
+# navigation surface.
+html_theme_options = {
+    "show_toc_level": 3,
+    "show_navbar_depth": 2,
+    "use_sidenotes": True,
+    "primary_sidebar_end": [],
+}
+html_sidebars = {
+    "**": [],  # empty left sidebar — unified TOC lives on the right
+}
+
 # Suppress warnings for missing references
 suppress_warnings = ["ref.doc"]
 

docs/_source/requirements.txt

@@ -1,3 +1,3 @@
 # Sphinx documentation build dependencies for Read the Docs
 sphinx>=8.0
-sphinx-rtd-theme>=3.0
+sphinx-book-theme>=1.1