Bump Sphinx to 8.2.3 to satisfy myst-parser 5.0.0

myst-parser 5.0.0 requires sphinx>=8,<10, but docs/requirements.txt
was still pinning Sphinx==7.4.7, so pip couldn't resolve the
environment and the Read the Docs build failed before sphinx-build
ran. Bumping to the latest 8.x resolves the conflict.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: kyleconroy

docs/requirements.txt

@@ -2,7 +2,7 @@ Babel==2.18.0
 Jinja2==3.1.6
 MarkupSafe==3.0.3
 Pygments==2.20.0
-Sphinx==7.4.7
+Sphinx==8.2.3
 certifi==2026.2.25
 chardet==7.4.3
 commonmark==0.9.1