docs: pin sphinx_rtd_theme to exact commit hash

Pin the sphinx_rtd_theme dependency from a branch reference to a
specific commit hash for supply chain safety (incident-51987).

Signed-off-by: Sean Meyer <sean.meyer@datadoghq.com>

Author: SeanMeyer

Documentation/requirements-min/requirements.txt

@@ -2,7 +2,7 @@ sphinx==7.1.2
 sphinx-autobuild==2021.3.14
 
 # Custom theme, forked from Read the Docs
-git+https://github.com/cilium/sphinx_rtd_theme.git@cilium/rebase-2023-09#egg=sphinx-rtd-theme-cilium
+git+https://github.com/cilium/sphinx_rtd_theme.git@77d60dbce4cc93358fa6156cbafd7c49214b3c89#egg=sphinx-rtd-theme-cilium
 
 # We use semver to parse Cilium's version in the config file
 semver==3.0.4