Pin Sphinx dependencies for m2r2 compatibility

m2r2 has specific version requirements:
- mistune==0.8.4 (for BlockGrammar class, removed in 2.0+)
- docutils==0.16 (for reprunicode attribute)
- sphinx<7.0 (for docutils compatibility)

Earlier attempts to fix this incorrectly focused on sphinx_rtd_theme
versions or tried newer versions of docutils/mistune, but m2r2
specifically requires these older versions due to API dependencies.

[claude-3.5-sonnet in Cursor]

docs/requirements.txt

@@ -1,6 +1,9 @@
+sphinx<7.0
+docutils==0.16
+mistune==0.8.4
+m2r2
 -e .
 numpydoc
-m2r2
 sphinxcontrib.mermaid
 sphinx_rtd_theme
 sphinxcontrib-apidoc