Add cc-by-sa logo to footer

_templates/cc-by-sa.html

+<a href="https://creativecommons.org/licenses/by-sa/4.0/">
+    <img src="https://mirrors.creativecommons.org/presskit/buttons/88x31/svg/by-sa.svg"/>
+</a>

conf.py

@@ -153,6 +153,7 @@ html_theme_options = {
     "navbar_end": ["theme-switcher", "navbar-icon-links"],
     # "navbar_persistent": ["search-button"],
     "footer_start": ["copyright"],
+    "footer_center": ["cc-by-sa"],
     "footer_end": ["last-updated"],
     # "content_footer_items": ["last-updated"],
     "secondary_sidebar_items": {