Rename: `files/` ▶︎ `static/`, `documents/` ▶︎ `render/`

[Zearin]

Elsewhere, DocPad is now using static/ instead of files/ for clarity. (After all, all of DocPad consists of “files”. ☺)

[Zearin]

@balupton I don’t have push access to this repo. Can you help?