docs: add introductory info to bootstrapping guide
docs: add introductory info to bootstrapping guide

Also: expand "Unix-like" systems to a few examples

RELNOTES: none
PiperOrigin-RevId: 213285215

davidzchen/bazel,perezd/bazel,ulfjack/bazel,twitter-forks/bazel,twitter-forks/bazel,ButterflyNetwork/bazel,perezd/bazel,perezd/bazel,safarmer/bazel,akira-baruah/bazel,akira-baruah/bazel,katre/bazel,cushon/bazel,safarmer/bazel,meteorcloudy/bazel,dslomov/bazel-windows,ulfjack/bazel,werkt/bazel,meteorcloudy/bazel,bazelbuild/bazel,bazelbuild/bazel,bazelbuild/bazel,dslomov/bazel-windows,dslomov/bazel,safarmer/bazel,dslomov/bazel,dslomov/bazel,ButterflyNetwork/bazel,werkt/bazel,meteorcloudy/bazel,perezd/bazel,ulfjack/bazel,twitter-forks/bazel,twitter-forks/bazel,safarmer/bazel,cushon/bazel,werkt/bazel,davidzchen/bazel,davidzchen/bazel,perezd/bazel,aehlig/bazel,aehlig/bazel,katre/bazel,dslomov/bazel,akira-baruah/bazel,meteorcloudy/bazel,katre/bazel,akira-baruah/bazel,meteorcloudy/bazel,katre/bazel,ButterflyNetwork/bazel,dslomov/bazel,davidzchen/bazel,twitter-forks/bazel,dslomov/bazel,ulfjack/bazel,cushon/bazel,akira-baruah/bazel,safarmer/bazel,ButterflyNetwork/bazel,katre/bazel,werkt/bazel,ulfjack/bazel,ulfjack/bazel,werkt/bazel,bazelbuild/bazel,aehlig/bazel,perezd/bazel,davidzchen/bazel,safarmer/bazel,twitter-forks/bazel,akira-baruah/bazel,twitter-forks/bazel,bazelbuild/bazel,meteorcloudy/bazel,aehlig/bazel,perezd/bazel,bazelbuild/bazel,aehlig/bazel,dslomov/bazel-windows,aehlig/bazel,meteorcloudy/bazel,dslomov/bazel,dslomov/bazel-windows,ulfjack/bazel,aehlig/bazel,dslomov/bazel-windows,dslomov/bazel-windows,ButterflyNetwork/bazel,cushon/bazel,ButterflyNetwork/bazel,werkt/bazel,cushon/bazel,katre/bazel,davidzchen/bazel,cushon/bazel,davidzchen/bazel