docs: Fix issue in readthedocs builds.

Somehow due to some dependency conflict resolution the Read The
Docs build pipeline ended up installing the rtd sphinx theme
with v0.5 instead of a newer version, which was somehow incompatible
with the newer sphinx releases.

docs/requirements.txt

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