docs: rename "doc" directory to "docs"

Also remove `CONTRIBUTING.adoc` and `FAQ.adoc` from project root since
GitHub knows to pick them up from the docs directory.

Fixes #2273

default.nix

@@ -1,7 +1,7 @@
 { pkgs ? import <nixpkgs> { } }:
 
 rec {
-  docs = with import ./doc { inherit pkgs; }; {
+  docs = with import ./docs { inherit pkgs; }; {
     html = manual.html;
     manPages = manPages;
     json = options.json;