Set doc contribution link to repository

[bjrne]

There is no obvious way to contribute to the documentation webpage on the page itself, so the link under the "contribution" section should link to the documentation repository.

[jackycute]

Hi @bjrne,

Thanks for the reminder.
The current best way to contribute to the doc is leaving comment/suggest edit directly at the https://hackmd.io/c/codimd-documentation, the github repo codimd-docs is used for backup at the moment.