Update handbook with documentation on blog only review apps

Why is this change being made?

Changes have been made to our build process to allow for blog only review apps. Blog only review apps are partial builds of about.gitlab.com and result in review apps being available in 7 minutes.

Blog only review apps include the following pages:

• about.gitlab.com
• about.gitlab.com/blog and all subsequent blog post pages
• about.gitlab.com/events/commit and all subsequent commit pages

How to trigger a blog only review app?

To trigger a blog only review app, simply place "Blog post:" in the MR title

What this MR does?

This MR updates handbook documentation on how to trigger blog only review apps.

This is a follow up MR to !41311 (merged)

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 #6355 (closed)