Auto-generate digest of blog posts on website homepage

Currently, the list of blog post titles and their descriptions that appear on the Mesos homepage is updated manually, requiring an individual to update the news section of `source/index.html.md` manually. This should instead dynamically update and display the most-recent 5 blog posts.

This change will require editing the website. Proposed patches may be attached to this ticket. It may also require changing the website homepage from a .md file to .erb in order to execute ruby code.

As an extra, additional metadata may be useful to display including release information (links to release notes, a summary of the post for display purposes, and blog post announcement) – tickets tracking this additional metadata include MESOS-2440, MESOS-2441, and MESOS-2442.