Add version picker in the navbar

Fixes: #951
Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>

changelogs/internal/newsfragments/2256.clarification

@@ -0,0 +1 @@
+Add version picker in the navbar.

config/_default/hugo.toml

@@ -78,6 +78,10 @@ current_version_url = "https://spec.matrix.org/latest"
 # major = "1"
 # minor = "16"
 
+[[params.versions]]
+# We must include this parameter to enable docsy's version picker in the navbar. The picker
+# is populated automatically in navbar-version-selector.html.
+
 # User interface configuration
 [params.ui]
 # Collapse HTTP API and event <details> elements

layouts/_partials/navbar-version-selector.html

@@ -0,0 +1,28 @@
+{{- /*
+
+  A version of the navbar-version-selector.html partial in Docsy,
+  modified to read the versions from changelog child pages so that
+  we don't have to maintain the menu separately in hugo.toml.
+
+*/ -}}
+
+{{ $changelog := site.GetPage "changelog" }}
+{{ $pages := $changelog.RegularPages.ByDate.Reverse }}
+
+<div class="dropdown">
+    <a class="nav-link dropdown-toggle" href="#" role="button" data-bs-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
+        All Versions
+    </a>
+    <ul class="dropdown-menu">
+        {{- /* Prepend a link to the unstable spec version  */ -}}
+        <li><a class="dropdown-item" href="https://spec.matrix.org/unstable">Unstable</a></li>
+        {{- /* Add a link for each vX.X version  */ -}}
+        {{ range $pages }}
+            {{ if findRE `^v[0-9]+\.[0-9]+$` .Params.linkTitle }}
+                <li><a class="dropdown-item" href="https://spec.matrix.org/{{ .Params.linkTitle }}">{{ .Params.linkTitle }}</a></li>
+            {{ end }}
+        {{ end }}
+        {{- /* Append a link to historical spec versions in the changelog  */ -}}
+        <li><a class="dropdown-item" href="/changelog/historical/">Historical</a></li>
+    </ul>
+</div>