Removed unused variables from "docs/Makefile".
Removed unused variables from "docs/Makefile".

They actually have been broken since 00d8049418cf,
where NXT_VERSION was removed from nxt_main.h.

Also, shebang is added.

nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit