doc: remove remnants of docbook times

all xml-related tooling can go away. shell.nix is no longer useful since
the makefile is gone and the build runs entirely via a derivation, and
gitignore is thus also no longer that useful. it may filter out some
swap files, but its main reason to exist (keeping generated files out of
a concurrent build of the derivation) has gone away.

doc/shell.nix

@@ -1,3 +0,0 @@
-{ pkgs ? import ../. { } }:
-(import ./default.nix { }).overrideAttrs
-(x: { buildInputs = (x.buildInputs or [ ]) ++ [ pkgs.xmloscopy pkgs.ruby ]; })