add abt node description

[robroyhobbs]

@wangshijun do you want me to add a description for node here https://github.com/ArcBlock/abtnode-docs/blob/master/src/introduction/what-is-abt-node/index.md. I feel that we are actually missing what the node is all about. Even though its just documentation we still need to provide the context of what it is, etc.