docs: Fallback to classic theme if furo is not available

Signed-off-by: Free Ekanayaka <free@ekanayaka.io>

docs/conf.py

@@ -1,3 +1,6 @@
+import sphinx
+import os.path
+
 # The master toctree document.
 master_doc = 'index'
 
@@ -24,7 +27,10 @@
 #html_theme = 'material'
 # html_theme = 'material'
 #html_theme = 'furo'
-html_theme = 'furo'
+if os.path.exists(os.path.join(os.path.dirname(os.path.dirname(sphinx.__file__)), "furo")):
+    html_theme = 'furo'
+else:
+    html_theme = 'classic'
 html_theme_path = ['_themes']
 
 # Theme options are theme-specific and customize the look and feel of a theme