Convert English docs to markdown (#1180)

- All English docs are written in markdown now.
  HTML is only used when it is required for theming or for elements that have no markdown counterpart.

- Fix a bug where pagination is not shown
  The pagination used the "||" operator that does not exist in Liquid, "or" does.

- Markdown parser set to Redcarpet
  The default Markdown parser Maruku is not able to build the documentation and throws an error.