gitlab.md

diagnostics	line column message 3 1 unsupported TOC directive `[[_TOC_]]`; mdsmith has no heading TOC equivalent; use `<?catalog?>` for file indexes (MDS019)

GitLab-flavored TOC directive

[[TOC]]

GitLab Flavored Markdown and Azure DevOps expand this into a TOC; CommonMark and goldmark render it as plain text.