docs: Update CommBlog publishing instructions for event reports
Previously, the docs asked individuals to sign up on the CommBlog
mailing list and request a review on the list. However, the mailing list
was deprecated and reviews now happen on Discourse.
However, this might change again as @bcotton was considering to move
this a Pagure ticket-based system. To future-proof this change, I
changed the instructions to point at the CommBlog page on how to write
an article. This way, if @bcotton moves to Pagure for handling reviews,
these docs shouldn't need to be updated again.