Import guide content to Drupal.org via Zensical importer

In #3470837: Decide on the future of the user guide we converted the entire user guide content to Markdown, and configured it to be built with Zensical.

Now, we need to get things wired up so that the Zensical/Markdown content is imported to Drupal.org in-place of the old asciidoc content.

The existing asciidoc importer generates these pages https://www.drupal.org/docs/user_guide/en/index.html. We'll probably want to either mirror the path names, or make sure we end up with redirects.

One thing to be aware of is that the repo has a script that runs before a Zensical import that re-writes URLs. This allows us to replicate the asciidoc feature where you could link to a document by it's ID like <install-setup>. The guide's content has a convention where Markdown links like [file-stem](./path/to/file-stem.md) are updated by https://git.drupalcode.org/project/user_guide/-/blob/11.x/scripts/zensic... so that [file-stem] gets replaced with the actual title of the page that's being linked.

Remaining tasks ... TBD.