doc: Add cockpit-version from playbook

We had set the cockpit-version attribute from the antora file which is
used by playbooks. This makes the attribute become locked and can't be
modified beforehand (that easily).

Proper usage, for us at least, is to update the cockpit-version through
an extension in cockpit-docs repo instead. This is only really a hack we
have to do due to publishing through tags instead of branches.

Signed-off-by: Freya Gustavsson <freya@venefilyn.se>

antora-playbook.yml

@@ -13,3 +13,6 @@ ui:
     snapshot: true
 output:
   dir: ./doc/output/antora
+asciidoc:
+  attributes:
+    cockpit-version: latest+git

doc/antora.yml

@@ -2,10 +2,6 @@ name: cockpit-guide
 version: true
 title: Cockpit
 start_page: guide:index.adoc
-asciidoc:
-  attributes:
-    # Set by CLI option --attribute=cockpit-version=123
-    cockpit-version: latest
 nav:
 - modules/guide/nav.adoc
 - modules/man/nav.adoc