docs: pin theme version

rtd.io uses a severely outdated theme version with several known bugs by default. Pin to a never version to get the bugfixes.

docs: pin theme version
32ae0719 · Matthias Schiffer · 028c1de0 · 32ae0719
Unverified Commit 32ae0719 authored 3 years ago by Matthias Schiffer
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
-sphinx-rtd-theme
+sphinx-rtd-theme==0.5.2