doc: add sphinx_rtd_theme extension

Newer versions of sphinx_rtd_theme require to add sphinx_rtd_theme to the
list of extensions. Cf.
readthedocs/readthedocs.org#9654

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/conf.py

@@ -228,7 +228,7 @@
 try:
     import sphinx_rtd_theme
     html_theme = 'sphinx_rtd_theme'
-    html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
+    extensions.append('sphinx_rtd_theme')
 except ImportError:
     sys.stderr.write('Warning: The Sphinx \'sphinx_rtd_theme\' HTML theme was not found. Make sure you have the theme installed to produce pretty HTML output. Falling back to the default theme.\n')