Support index page for each folder

[jbmoelker]

Currently folders do not have an index page. I would like them to so we have a central place per folder to introduce its contents. Also each folder translates to a /folder-url-part/ which you would expect to have an index. As an editor I would like to be able to give an index page a distinctive title in Dropbox Paper, rather than having to call them each Index or Readme. So a option could be prefixing the document you want to become the index page of its folder, with for instance __ or an index emoji (📇).