Merge pull request #241656 from mobusoperandi/update-doc-contributing

fix outdated contributing to documentation

doc/contributing/contributing-to-documentation.chapter.md

···
 
 Additional syntax extensions are available, all of which can be used in NixOS option documentation. The following extensions are currently used:
 
+- []{#ssec-contributing-markup-tables}
+  Tables, using the [GitHub-flavored Markdown syntax](https://github.github.com/gfm/#tables-extension-).
+
 - []{#ssec-contributing-markup-anchors}
   Explicitly defined **anchors** on headings, to allow linking to sections. These should be always used, to ensure the anchors can be linked even when the heading text changes, and to prevent conflicts between [automatically assigned identifiers](https://github.com/jgm/commonmark-hs/blob/master/commonmark-extensions/test/auto_identifiers.md).