docs: Stop linking to specific versions of our docs entirely.

We started doing this for install docs in de2a2d0df, because `latest`
wasn't suitable and because I didn't know about readthedocs's `stable`
feature.  The result has been that even with a checklist item, we
don't reliably update the link.

Instead, use the special `stable` version identifier on readthedocs to
link automatically to the highest version it knows about.

docs/subsystems/release-checklist.md

@@ -53,6 +53,4 @@ preparing a new release.
   to master.
 * Update `ZULIP_VERSION` in `version.py`, and `release` and `version` in
  `docs/conf.py`, to e.g. `1.6.0+git`.
-* Update the handful of places where we link to docs for the latest
-  release, rather than for master.  See `git grep 'zulip.readthedocs.io/en/[0-9]'`.
 * Consider removing a few old releases from ReadTheDocs.

templates/zerver/plans.html

@@ -111,7 +111,7 @@
                                     <div class="pricing-details">
                                         Free and open source forever
                                     </div>
-                                    <a href="https://zulip.readthedocs.io/en/1.8.0/prod.html">
+                                    <a href="https://zulip.readthedocs.io/en/stable/prod.html">
                                         <button class="green" type="button">
                                             Install a Zulip server
                                         </button>