SPEC: promote Pods to top-level concept
SPEC: promote Pods to top-level concept

Following some of the discussion in #207, this is an attempt to
restructure the spec by promoting the Pod to a top-level concept (along
with ACIs, image discovery, and ACE). Decoupling them from the executor
section makes it clearer that producing/manipulating pods can be
completely independent of execution.

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