Generate the release post from items in X_Y directory

Why is this change being made?

The release post is currently created by assembling all the items into a single YAML file, but the release post items are themselves drafted as individual files, and rendered as a preview release post from the individual files. The process of reviewing and finalizing the release process needs to happen in 4 days (18th to 22nd), and should be focussed on the quality of the content, not copying and pasting text file to file.

The commit adds support for using the directory data/release_posts/X_Y for release post items, rendering them in a similar way to the unreleased items are shown in a preview kind of release post.

This means the release post manager, need only move the files from the unreleased directory to the X_Y directory to add content to the release post.

Closes #6316 (closed)

/cc @gl-product

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