Wiki Table of Contents - allow exclude entry

In wiki add tag/flag to exclude an entry from the Table of Contents (e.g. add <!-- NOTOC --> to a header line to skip it.)
Otherwise page titles that go before TOC are always included n TOC.

Discussed in gitlab-foss#2494 (closed), @rspeicher suggested to open as new proposal.

This feature is supported in Gitlab's own "About Gitlab" https://about.gitlab.com/handbook/product/technical-writing/markdown-guide/#table-of-contents-toc with {:.no_toc}