Manually maintaining the table of contents relies on how HTML fragments
are generated, it is prone to errors (infact it is already out of sync)
and just plain boring.

Unfortunately there is no common agreement on all markdown flavours on
how the table of contents must be implemented. Here the `[[_TOC_]]` tag
has been used because it is supported by both GitLab and GitHub:

    https://gitlab.com/gitlab-org/gitlab/-/issues/21901