Refactor the Drupal User Guide to use Markdown and Zensical

Issue: #3470837

Replace the previous AsciiDoc-based toolchain with one that uses Zensical. One site per locale under content/ to maintain translations in aligment with how Zensical works.

Migrate the guide content and contributor docs to Markdown, and maintain existing features as best as possible. There are some features of asciidoc that Markdown doesn't support. And for now, this doesn't include publishing a PDF or Ebook though we can add that back if desired.

Update GitLab Pages CI, templates, and authoring guidance for the new workflow, remove the legacy source/build stack that is no longer needed.

Closes #3470837