Clean up any leftover Nanoc/migration code

Drop any leftover Nanoc references from other projects (gitlab, GDK, etc), and remove migration scripts from gitlab-docs-hugo.

• Sarah: Migration scripts and CI jobs in gitlab-docs-hugo
• Evan: Custom markdownlint rules in gitlab (replaced by shortcode validation)
• Evan: GDK integration for gitlab-docs (and any migration-specific handling for gitlab-docs-hugo)
• Evan: Nanoc review apps