Add missing CSS class to release post sections

Why is this change being made?

This MR adds the CSS class .release-post-section to the primary and secondary feature sections of the release post. It also adds the top-feature class to the top feature section as well as a .top-feature.release-post.section specific CSS modification to max-width. This is needed to ensure consistent presentation and formatting of the release contents.

Note that while this was discovered as part of the 15.2 release post process, this issue is currently live on GitLab.com and affects all release posts. It is not unique to 15.2.

Screenshots:

Top feature

Primary feature

Secondary feature

Author Checklist

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