Also cache collector output of main branch

Since dependency updates to the `docs-site` branch are frequent enough.

Author: marcphilipp

antora-playbook.yml

@@ -1,8 +1,6 @@
 antora:
   extensions:
     - require: './extensions/collector-cache.js'
-      excludes:
-        - main
     - '@antora/collector-extension'
     - require: '@antora/html-single-extension'
       config_file: ./antora-assembler-html.yml