Created a blog post for how we load merge request diffs

Closes #11413 (comment 556984515)

Checklist for writer

• Link to issue added, and set to close when this MR is merged
• Add disclaimer copy to the top of your blog post file: https://about.gitlab.com/handbook/marketing/blog/unfiltered/#link-to-disclaimer
• Blog post file formatted correctly, including any accompanying images
• unfiltered category entered in frontmatter
• Review app checked for any formatting issues
• Reviewed by fellow team member
• Filename updated to reflect current date (see https://about.gitlab.com/handbook/marketing/blog/#making-changes-to-your-blog-post)
• Assign to someone on your team with maintainer access to merge (no need to assign to an Editorial team member)