Repair diagram by taking it out of the list

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What does this MR do?

I noticed this Kramdown warning, and wanted to fix it:

kramdown warning(s) for <Nanoc::Core::CompilationItemRepView item.identifier=/ee/architecture/blueprints/cells/infrastructure/index.md name=default>
  Found invalidly used HTML closing tag for 'details' on line 302 - ignoring it
  Found no end tag for 'gcp' on line 282 - auto-closing it

      create  [0.04s]  public/ee/architecture/blueprints/cells/impacted_features/your-work.html
      create  [0.07s]  public/ee/architecture/blueprints/cells/index.html

In doc/architecture/blueprints/cells/infrastructure/index.md, there was a diagram, wrapped in a <details> tag, as part of an unordered list. This MR tries to remove SOME of the wrappers by taking out the unordered list. Maybe the diagram will render with this fix?

Steps to reproduce

1. Go to https://docs.gitlab.com/ee/architecture/blueprints/cells/infrastructure/index.html#architecture
2. Twirl down KAS: Select to Expand
3. See raw Mermaid markup.

Related issues

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