[doc] Add missing doc about pmd.github.io

[adangel]

• Describe, how to create new blog posts, that appear at https://pmd.github.io/#news
  • https://github.com/pmd/pmd.github.io/tree/master/_posts
  • in project management documented as a process: Describe the magic behind pmd.github.io
  • also the relationship to the release process
• As a first example, create a blog post about https://gearset.com/blog/helping-sf-developers-create-readable-and-maintainable-apex-code
  • We have this currently hidden in https://pmd.github.io/latest/pmd_projectdocs_trivia_news.html