fix: pin sphinx-rtd-theme to fix missing jquery lib

JQuery was removed as of Sphinx 6. The maintainers of our theme already
reacted to this but the release is not final yet.
Ref sphinx-doc/sphinx#10070
Ref readthedocs/sphinx_rtd_theme#1385

Signed-off-by: Richard Steinmetz <richard@steinmetz.cloud>

Pipfile

@@ -7,7 +7,7 @@ verify_ssl = true
 
 [packages]
 rst2pdf = "*"
-sphinx-rtd-theme = "*"
+sphinx-rtd-theme = "==1.2.0rc2"
 Sphinx = "*"
 Pillow = "*"