feat(Alert): Adding error alert example back to variants

What does this MR do?

Adding the error example back to the variants, but this time at the bottom so that its sticky position doesn’t mess with the rest.

This allows all alert variants to easily be referenced in Pajamas in one place gitlab-org/gitlab-services/design.gitlab.com!2206 (diffs).

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [-] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Security

N/A