add initial galley intro to "what is istio" concept page (#2113)
add initial galley intro to "what is istio" concept page (#2113)


istio/istio.io,istio/istio.io,istio/istio.io,linsun/istio.github.io,linsun/istio.github.io,linsun/istio.github.io,istio/istio.io,geeknoid/istio.github.io,istio/istio.io,istio/istio.io,istio/istio.io,linsun/istio.github.io,linsun/istio.github.io,geeknoid/istio.github.io,geeknoid/istio.github.io,geeknoid/istio.github.io,geeknoid/istio.github.io