Add automated schema markup blog posts

Why is this change being made?

This MR adds the generate_blog_post_schema function to helpers/blog_helper.rb file to create an automated schema for blog posts' scham markups. It takes a post as argument and then returns a json object that is then used in the slippers-blog-post.erb template file as an application/ld+json script.

This helper makes sure that a schema markup is generated for every blog post in the www repo

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Closes gitlab-com/marketing/digital-experience/buyer-experience#2014 (closed)