[docs] Add links to release based runtimes

We have the release based runtimes documented in this repo and
published.  However, The link to them from within the PTI page is
easily missed.  This change adds a specific runtimes index page and
links to that from the reference section of the governance homepage.

The list included in the PTI page uses the default ascii sorting i.e.
2023.1, 2023.2, 2024.1, Stein, ..., Zed

The change also updates this list to follow the release order.

Change-Id: I0b8cd23986f6abb168486f8cf2a8c30e2a1b30e9

reference/index.rst

@@ -35,3 +35,4 @@ Reference documents which need to be revised over time.
    election-exceptions
    tc-chair-elections
    emerging-technology-and-inactive-projects
+   runtimes/index

reference/project-testing-interface.rst

@@ -221,5 +221,7 @@ The officially tested runtimes for each cycle can be found here:
 .. toctree::
    :maxdepth: 1
    :glob:
+   :reversed:
 
-   runtimes/*
+   runtimes/[stuvwxyz]*
+   runtimes/2*

reference/runtimes/index.rst

@@ -0,0 +1,20 @@
+========================
+ Release based Runtimes
+========================
+
+OpenStack is tested with certain runtimes and expected to work there.  For
+more details about what this means, refer to the
+:ref:`Project Testing Interface tested runtimes <pti-tested-runtimes>`.
+
+Below are the specific runtimes per release.
+
+..
+   TODO: The list below breaks with the 3000.1 release.
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+   :reversed:
+
+   [stuvwxyz]*
+   2*