docs: replace redirects with copies of files

This means that <https://martinvonz.github.io/jj/latest/index.html> will
not longer redirect to
<https://martinvonz.github.io/jj/v0.18.0/index.html> (or similar) after
the next release, as requested by @jyn514 on Discord. Instead, it will
be a copy.

Originally, the plan was to use mike's symlink functionality and make
`latest` a symlink. See a comment inline for how that didn't work; we
could do that after changing how GitHub Pages are built.

Before the next `jj` release, and after this PR is merged, you can play
with this by going to <https://martinvonz.github.io/jj/main/> which is
an alias for <https://martinvonz.github.io/jj/prerelease/>.

Because of previous commits in this PR, the latest version will be
marked in the version selector as "latest", so hopefully this won't
confuse users.

Due to the parent commit, the "latest" URL will be considered canonical.

.github/scripts/docs-build-deploy

@@ -17,6 +17,11 @@ export SOURCE_DATE_EPOCH=$(git log -1 --pretty=%ct docs/ mkdocs.yml)
 # https://github.com/python-poetry/poetry/issues/8623
 export PYTHON_KEYRING_BACKEND=keyring.backends.fail.Keyring
 poetry install --no-root # Only really needed once per environment unless there are updates
-# TODO(ilyagr): The new default "alias-type" is symlink, we should consider
-# switching to it.
-poetry run -- mike deploy --alias-type redirect "$@"
+# TODO: `--alias-type symlink` is the
+# default, and may be nicer in some ways. However,
+# this requires deploying to GH Pages via a "custom GitHub Action", as in
+# https://docs.github.com/en/pages/getting-started-with-github-pages/configuring-a-publishing-source-for-your-github-pages-site#publishing-with-a-custom-github-actions-workflow.
+# Otherwise, you get an error:
+# > Site contained a symlink that should be dereferenced: /main.
+# > For more information, see https://docs.github.com/github/working-with-github-pages/troubleshooting-jekyll-build-errors-for-github-pages-sites#config-file-error.
+poetry run -- mike deploy --alias-type copy "$@"