Show documentation page actions when source exists

Simplification, there's no need to rely on html_show_sourcelink Sphinx configuration

Author: m-aciek

Doc/tools/templates/customsourcelink.html

@@ -1,4 +1,4 @@
-{%- if show_source and page_source_suffix is defined %}
+{%- if page_source_suffix is defined %}
   <script>
     document.addEventListener('DOMContentLoaded', () => {
         const title = document.querySelector('meta[property="og:title"]').content;