Bump sbt-microsites to 1.3.0 (#107)

Several changes were required:
* micrositeCompilingDocsTool removed in 1.3.0 (always mdoc)
* micrositeExtraMdFilesOutput seems to require specifically being set
  (maybe changed default?)
* micrositeTheme must be set, "pattern" matches old theme
* colliding index.md caused issue, but was unused so deleted
* Contributors.scala needed to be updated for changes to github4s
* "position" key should be only be present on markdown files that should
  be present as a link on the home page menu