Makefile: Build docs via dummy target in postcheck
Makefile: Build docs via dummy target in postcheck

This is intended to encourage developers to keep the docs up to date and
clean, rather than hiding documentation bugs behind a CI system
somewhere.

This target uses the sphinx "dummy" builder which only parses the docs
and checks for consistency, but avoids actually creating output. There's
one limitation with this dummy builder: It doesn't support the tabs
assets which we use for YAML/JSON selection of policies. Typically this
causes the following warning to be printed, which we use grep to avoid:

  WARNING: Not copying tabs assets! Not compatible with dummy builder

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@covalent.io>

scanf/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,scanf/cilium,eloycoto/cilium,scanf/cilium,michi-covalent/cilium,eloycoto/cilium,scanf/cilium,michi-covalent/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,eloycoto/cilium,tgraf/cilium,cilium/cilium,eloycoto/cilium,tgraf/cilium,scanf/cilium,eloycoto/cilium,eloycoto/cilium,eloycoto/cilium,scanf/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,scanf/cilium,michi-covalent/cilium,tgraf/cilium,cilium-team/cilium,tklauser/cilium,cilium-team/cilium