Merge branch 'master' into remove-extra_css_files

`extra_css_files` was add in https://github.com/rtfd/readthedocs.org/commit/e759c50697c5c9b210f5b7ea2978e24b25aef901 but serves no purpose (not sure why it was added intially) users should use `Sphinx.add_stylesheet` instead.

sphinx_rtd_theme/layout.html

@@ -36,11 +36,6 @@
     {%- endif %}
   {%- endfor %}
 
-  {# "extra_css_files" is a theme option and it's always a string #}
-  {%- for css_file in extra_css_files %}
-    <link rel="stylesheet" href="{{ pathto(css_file, 1)|escape }}" type="text/css" />
-  {%- endfor -%}
-
   {#- FAVICON
       favicon_url is the only context var necessary since Sphinx 4.
       In Sphinx<4, we use favicon but need to prepend path info.