Add {{ .TableOfContents }} support in asciidoc

[odknt]

Fixes #1687

helpers.ExtractTOC() processes only TOC in output by blackfriday now.
This PR is finds and replaces TOC in output of asciidoctor with blackfriday compatible TOC.

[stale]

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[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.