Update documentation for blog and release posts

Why is this change being made?

We are splitting the blog into two separate Middleman blogs, the marketing blog and release blog. This changes where blog posts and release posts are located in the repository.

This the first iteration that updates blog documentation with new directory information. Essentially, blog posts now reside in /source/blog/posts and release posts reside in /source/releases/posts. That change is completed in this MR.

We should merge !37470 (merged) before merging this MR.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what

Related #6120 (closed)