In the web bundle, rename the editable file to make it more self-documenting and intuitive

[joanise]

Based on user feedback, "editable" is confusing. Indeed, the format is pretty opaque, while the multi file format is much more manually editable. What we mean is compatible with our editor, but that's not clear.

Desiderata

• since it's the same file you get when you pick Offline HTML as the download option, make its name consistent with that (e.g., maybe Offline-HTML/slugified-title.html?)
  • make the connection to #299 point about "Editor interface: make it clearer where the file to upload comes from"

[joanise]

Current readalong folder with multi-file format: www