Update Skylark documentation
Update Skylark documentation

- Add debugging tips
- Give more visibility to tools/build_defs/g3doc/
- Mention Standalone Skylark
- Remove some mentions of build extensions

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MOS_MIGRATED_REVID=137175010

zhexuany/bazel,kchodorow/bazel-1,kchodorow/bazel,perezd/bazel,werkt/bazel,bazelbuild/bazel,zhexuany/bazel,juhalindfors/bazel-patches,mbrukman/bazel,spxtr/bazel,katre/bazel,bazelbuild/bazel,davidzchen/bazel,meteorcloudy/bazel,safarmer/bazel,werkt/bazel,aehlig/bazel,twitter-forks/bazel,dslomov/bazel,kchodorow/bazel,meteorcloudy/bazel,werkt/bazel,kchodorow/bazel-1,katre/bazel,damienmg/bazel,kchodorow/bazel-1,davidzchen/bazel,dropbox/bazel,Asana/bazel,kchodorow/bazel,damienmg/bazel,perezd/bazel,ulfjack/bazel,davidzchen/bazel,ulfjack/bazel,hermione521/bazel,ulfjack/bazel,mbrukman/bazel,variac/bazel,safarmer/bazel,spxtr/bazel,cushon/bazel,twitter-forks/bazel,snnn/bazel,mikelikespie/bazel,Asana/bazel,aehlig/bazel,dslomov/bazel-windows,damienmg/bazel,cushon/bazel,kchodorow/bazel-1,dropbox/bazel,safarmer/bazel,zhexuany/bazel,dslomov/bazel,kchodorow/bazel,dslomov/bazel,snnn/bazel,hermione521/bazel,davidzchen/bazel,dropbox/bazel,damienmg/bazel,mikelikespie/bazel,meteorcloudy/bazel,LuminateWireless/bazel,snnn/bazel,LuminateWireless/bazel,twitter-forks/bazel,cushon/bazel,aehlig/bazel,ulfjack/bazel,dslomov/bazel-windows,bazelbuild/bazel,meteorcloudy/bazel,meteorcloudy/bazel,LuminateWireless/bazel,werkt/bazel,mikelikespie/bazel,damienmg/bazel,spxtr/bazel,LuminateWireless/bazel,ButterflyNetwork/bazel,juhalindfors/bazel-patches,perezd/bazel,bazelbuild/bazel,ButterflyNetwork/bazel,hermione521/bazel,snnn/bazel,akira-baruah/bazel,mikelikespie/bazel,dslomov/bazel,kchodorow/bazel,Asana/bazel,dropbox/bazel,zhexuany/bazel,twitter-forks/bazel,ButterflyNetwork/bazel,katre/bazel,safarmer/bazel,juhalindfors/bazel-patches,meteorcloudy/bazel,juhalindfors/bazel-patches,Asana/bazel,katre/bazel,mbrukman/bazel,dslomov/bazel-windows,dropbox/bazel,bazelbuild/bazel,damienmg/bazel,spxtr/bazel,akira-baruah/bazel,hermione521/bazel,dslomov/bazel-windows,davidzchen/bazel,dslomov/bazel,mbrukman/bazel,akira-baruah/bazel,ulfjack/bazel,dslomov/bazel-windows,variac/bazel,perezd/bazel,kchodorow/bazel,twitter-forks/bazel,juhalindfors/bazel-patches,Asana/bazel,katre/bazel,variac/bazel,dslomov/bazel,Asana/bazel,katre/bazel,perezd/bazel,hermione521/bazel,davidzchen/bazel,kchodorow/bazel-1,snnn/bazel,zhexuany/bazel,ulfjack/bazel,aehlig/bazel,spxtr/bazel,aehlig/bazel,werkt/bazel,snnn/bazel,safarmer/bazel,cushon/bazel,twitter-forks/bazel,LuminateWireless/bazel,ButterflyNetwork/bazel,davidzchen/bazel,hermione521/bazel,Asana/bazel,mbrukman/bazel,LuminateWireless/bazel,dslomov/bazel,variac/bazel,variac/bazel,juhalindfors/bazel-patches,damienmg/bazel,cushon/bazel,akira-baruah/bazel,twitter-forks/bazel,mbrukman/bazel,akira-baruah/bazel,kchodorow/bazel-1,cushon/bazel,akira-baruah/bazel,meteorcloudy/bazel,mikelikespie/bazel,variac/bazel,werkt/bazel,dslomov/bazel-windows,perezd/bazel,mikelikespie/bazel,bazelbuild/bazel,zhexuany/bazel,perezd/bazel,variac/bazel,spxtr/bazel,dropbox/bazel,ButterflyNetwork/bazel,safarmer/bazel,aehlig/bazel,snnn/bazel,spxtr/bazel,kchodorow/bazel,ulfjack/bazel,juhalindfors/bazel-patches,aehlig/bazel,ButterflyNetwork/bazel