docs: disable main and nightly aliases for prerelease version

This is an alternative to https://github.com/martinvonz/jj/pull/3732. I
think it's pretty important to show the "latest" alias. I don't know a
way to not show any alias for "prerelease" in the version selector
without deleting all of them. 

I'll have to then separately delete the directories for the aliases on
the gh-pages branch, so that the URL does not resolve to outdated pages.

The downsides are: this will break https://martinvonz.github.io/jj/main/... and
(only recently created) https://martinvonz.github.io/jj/nightly/... links. We
won't have an always-on demo of how well aliases work for when and if we adjust
how aliases (including the all-important latest alias) work in the future.

However, expanding on #3732 (comment), mike is now randomly alternating between
"nightly" and "main" being the first alias of prerelease (it never promised that
the set of aliases would be ordered). So, the version selector keeps switching
in which version it shows. Previously, this only affected the prerelease docs,
but now it also affects https://martinvonz.github.io/jj/latest/.

.github/workflows/docs.yml

@@ -29,6 +29,6 @@ jobs:
         run: |
           git config user.name 'jj-docs[bot]'
           git config user.email 'jj-docs[bot]@users.noreply.github.io'
-          .github/scripts/docs-build-deploy 'https://martinvonz.github.io/jj' prerelease nightly main --push
+          .github/scripts/docs-build-deploy 'https://martinvonz.github.io/jj' prerelease --push
       - name: "Show `git diff --stat`"
         run: git diff --stat gh-pages^ gh-pages || echo "(No diffs)"