fix: fix GFM markdown output

[aloisklink]

GFM output is currently broken (since v13.2.2), which prevents tables from being written to markdown.

For example, trying to use documentation ./__tests__/fixture/html/nest.input.js -f md will throw an Error: Cannot handle unknown node table` due to this GFM only syntax:https://github.com/documentationjs/documentation/blob/849a0836f4fbb4ecc8000c131882a4c8bf4cdc10/tests/fixture/html/nested.input.js#L173-L179

This is because as of remark v13.0.0, GFM support was moved to the remark-gfm pacakge.

https://github.com/documentationjs/documentation/commit/6b5bc2548db23fdab3fd8d29ac1aeedea7ad3a47 fixed parsing GFM, however, it didn't add GFM support for markdown output.

Fixes: f4a46b134f58639f352b3024801c3d98f901d66a