[nrf fromtree] doc: pin sphinx_rtd_theme version

We have recently been hit by an issue between docutils and
sphinx_rtd_theme. The problem is fixed since release 0.5.2. In order to
avoid similar problems in the future, pin the theme version.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

scripts/requirements-doc.txt

@@ -3,7 +3,7 @@
 breathe>=4.23.0
 docutils>=0.16
 sphinx>=3.3.0,<3.4.0
-sphinx_rtd_theme
+sphinx_rtd_theme==0.5.2
 sphinx-tabs
 sphinxcontrib-svg2pdfconverter