Add version switcher

Author: stevepiercy

docs/_static/switcher.json

@@ -0,0 +1,19 @@
+[
+    {
+        "name": "6 (stable)",
+        "version": "6.0",
+        "url": "https://6.docs.plone.org/"
+    },
+    {
+        "version": "5",
+        "url": "https://5.docs.plone.org/"
+    },
+    {
+        "version": "4",
+        "url": "https://4.docs.plone.org/"
+    },
+    {
+        "version": "3",
+        "url": "https://3.docs.plone.org/"
+    }
+]

docs/_templates/sections/sidebar.html

@@ -0,0 +1,4 @@
+{%- for sidebartemplate in sidebars %}
+{%- include sidebartemplate %}
+{%- endfor %}
+{%- include 'version-switcher.html' -%}

docs/conf.py

@@ -229,13 +229,25 @@
 # https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-html_use_opensearch
 html_use_opensearch = "https://6.docs.plone.org"
 
+html_sidebars = {
+    "**": [
+        "sidebar-logo.html",
+        "search-field.html",
+        "sbt-sidebar-nav.html",
+    ]
+}
+
 html_theme_options = {
     "path_to_docs": "docs",
     "repository_url": "https://github.com/plone/documentation",
     "repository_branch": "main",
     "use_repository_button": True,
     "use_issues_button": True,
     "use_edit_page_button": True,
+    "switcher": {
+        "json_url": "/_static/switcher.json",
+        "version_match": version,
+    },
     "extra_navbar": """
     <p class="ploneorglink">
         <a href="https://plone.org">