spec: Use {ext} also in simple discovery template.
spec: Use {ext} also in simple discovery template.

As simple discovery is just like meta discovery but with a predefined template,
to make it more consistent, use {ext} also in it.
Additionally specify that simple discovery doesn't provide a way to discover
Public Keys URLs.

Conflicts:
	SPEC.md

dgonyeo/spec,appc/spec,marineam/spec,3ofcoins/appc-spec,apcera/appc-spec,craimbert/spec,iaguis/appc-spec,apcera/appc-spec,tklauser/spec,yifan-gu/spec,stufflebear/spec,marineam/spec,iaguis/appc-spec,tklauser/spec,zenlinTechnofreak/spec,krnowak/appc-spec,jonboulle/spec,dgonyeo/spec,squeed/spec,endocode/appc-spec,zenlinTechnofreak/spec,ChengTiesheng/spec,iaguis/appc-spec,3ofcoins/appc-spec,endocode/appc-spec,jonboulle/spec,stufflebear/spec,appc/spec,appc/spec,ChengTiesheng/spec,craimbert/spec,marineam/spec,tklauser/spec,ChengTiesheng/spec,craimbert/spec,yifan-gu/spec,jonboulle/spec,alban/appc-spec,alban/appc-spec,dustymabe/spec,alban/appc-spec,dustymabe/spec,dustymabe/spec,apcera/appc-spec,dgonyeo/spec,squeed/spec,squeed/spec,3ofcoins/appc-spec,stufflebear/spec,endocode/appc-spec,yifan-gu/spec,krnowak/appc-spec,zenlinTechnofreak/spec,krnowak/appc-spec