Fix mixed use of tabs and spaces in indented blocks

4-space is our convention for indented blocks and should be used consistently.
The only exception is for C++ code which is tab-indented, as the engine code.

conf.py

@@ -53,7 +53,7 @@ import sphinx_rtd_theme
 html_theme = 'sphinx_rtd_theme'
 html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
 if on_rtd:
-	using_rtd_theme = True
+    using_rtd_theme = True
 
 # Theme options
 html_theme_options = {