Silence specified Kramdown warnings

What does this MR do and why?

Silence Kramdown warnings for Blueprint pages. These occasionally contain markup that's considered invalid Kramdown, but they render fine visually (e.g, throwing this warning: https://github.com/gettalong/kramdown/issues/387#issuecomment-285856504).

Kramdown warnings are manually corrected as part of TW monthly maintenance, and it would be better to focus on customer-facing content.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. On the main branch, compile Nanoc and watch the build process. You'll see errors like this when it reaches Blueprints:

  Found block HTML tag 'li' in span-level text on line 1
  Found invalidly used HTML closing tag for 'li' on line 1
  Found invalidly used HTML closing tag for 'ol' on line 1
  Found block HTML tag 'ol' in span-level text on line 1

3. Check out this branch, and compile again. Errors from Blueprint pages will no longer appear in the log.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.