docs: add what is hydra / what is hydra not section
docs: add what is hydra / what is hydra not section

leetal/hydra,ory-am/hydra,Bridg/hydra-1,ory/hydra,Bridg/hydra-1,ory/hydra,Bridg/hydra-1,ory-am/go-iam,ory/hydra,ory-am/go-iam,Grillz/hydra,Grillz/hydra,ory/hydra