Prepare documentation for publication

[weierophinney]

• Updates the documentation to use mkdocs.
• Re-organizes the documentation.
• Edits the documentation:
  • Edits for flow and content (fixes Check For Blockquotes In Docs #9, Check Documentation For Other Things #7, Check Documentation Code Blocks #6, and Check All Headers In Documentation #5).
  • Re-introduces the tables with options (lost during rST to Markdown conversion; fixes Check Documentation Tables #8).
• Creates automated documentation build chain.

Results are at https://zendframework.github.io/zend-tag/