Extract markdownit module

[Geminii]

Extract markdownit module :

• add PR for this on : feat!: update module markdownit-module#23
• add documentation single layout
• add test on the module

[Atinux]

Would you open a PR to remove the module?

[pi0]

Module is already extracted into nuxt-community/markdownit-module (preserving previous contributions). Would you please open a PR to update repo? (the ones here should be eventually removed)

[Geminii]

Module is already extracted into nuxt-community/markdownit-module (preserving previous contributions). Would you please open a PR to update repo? (the ones here should be eventually removed)

You're right, I didn't see that it already existed because I could have proposed a PR directly in this repo.
Thanks to point it. I will update this issue and associate a PR.