feat: add a cli product for the gt documentation

gt, the Gitea CLI, gets its own product next to the runner: the CLI entry
in the header, its own sidebars, search scope, crawler start url and
redirects, and `make cut-version PRODUCT=cli`. Until a release series is
listed, the develop docs are served at /cli/ without the unreleased banner.

A directory with an index page and no _category_.json now takes its
sidebar label and position from that page, which leads the group, so the
generated reference can be grouped by command.

Signed-off-by: bircni <[email protected]>

cli-docs_versions.json

@@ -0,0 +1 @@
+[]