Good question! You can complete the Writing on the Wall achievement by submitting a blog you've written on the Community Blog page. https://developers.mendix.com/community-blog/.
So, time to write something interesting/useful/insightful for the community!
I can answer one of your questions. At the bottom of every documentation page is a link to contribute to the documentation:
So points for that type of challenge are likely somewhat automated during the documentation update process.
Regarding a blog post, maybe someone from the Mendix Community Team can chime in and tell us how you would get points for that.
Hope that helps,