fix outdated contributing to documentation

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}

  [GFM tables](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).