docs: Fix documentation theme

This avoids failing builds with:
NameError: name 'html_theme' is not defined

It also makes the theme configuration explicit and correctly set up.

Signed-off-by: Andrei Gherzan <andrei@gherzan.com>

docs/conf.py

@@ -30,7 +30,10 @@
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
-extensions = ['myst_parser']
+extensions = [
+    'myst_parser',
+    'sphinx_rtd_theme'
+]
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
@@ -121,6 +124,7 @@ todo_include_todos = False
 # a list of builtin themes.
 #
 # html_theme = 'alabaster'
+html_theme = "sphinx_rtd_theme"
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the

docs/requirements.txt

@@ -1 +1,2 @@
 myst_parser
+sphinx_rtd_theme