doc: requirements: Update to Sphinx RTD Theme 3.0

Update to latest Sphinx RTD Theme version.
Main benefit will be to be able to leverage Sphinx 8.0 since previous
version of the theme was depending on 'sphinx < 8.0'.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>
diff --git a/doc/requirements.txt b/doc/requirements.txt
index b979eee..cb202de 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -1,7 +1,7 @@
 # DOC: used to generate docs
 
 sphinx
-sphinx_rtd_theme~=2.0
+sphinx_rtd_theme~=3.0
 sphinx-tabs
 sphinxcontrib-svg2pdfconverter
 pygments>=2.9