docs: Fix issue in readthedocs builds. (#806)

Somehow due to some dependency conflict resolution the Read The
Docs build pipeline ended up installing v0.5 of the rtd sphinx theme
instead of a newer version. This old version is incompatible
with newer sphinx releases, so setting the theme minimum version
to v1.

docs/requirements.txt

@@ -1,2 +1,2 @@
 sphinx<8
-sphinx_rtd_theme<2
+sphinx_rtd_theme>=1,<2