Reorganize toctree to make previous roadmaps less preeminent

This nests the historical roadmap under a specific roadmap/index
toctree, so that they don't all show up in the sidebar.

docs/index.rst

@@ -49,8 +49,10 @@ Tutorials
 Roadmap
 -------
 
-* :ref:`roadmap-2022`
-* :ref:`roadmap-2021`
+* Current roadmap: :ref:`roadmap-current`
+* Previous roadmaps
+
+  * :ref:`roadmap-2021`
 
 System Administration
 ---------------------
@@ -223,7 +225,7 @@ Indices and tables
    tutorials/index
    faq/index
    roadmap/roadmap-2022
-   roadmap/roadmap-2021
+   roadmap/index
    api-reference
    archive-changelog
    journal

docs/roadmap/index.rst

+Previous roadmaps
+-----------------
+
+This page lists our archived roadmaps. You can also have a look at our current roadmap: :ref:`roadmap-current`.
+
+.. toctree::
+   :maxdepth: 2
+   :titlesonly:
+
+   roadmap-2021

docs/roadmap/roadmap-2022.rst

+.. _roadmap-current:
 .. _roadmap-2022:
 
 Roadmap 2022