Move developer docs to GitHub Wiki?

These docs haven't been updated in a while (and probably don't need to be updated frequently). Moreover, in most cases, developers would be interested in the latest version of the code, so maintaining older versions seems to be quite an overhead. (Anyone who needs help with older versions could ask in the chatroom).

We could just move the docs and setup instructions to the Github Wiki section of the main gocd repository.

@arvindsv @nyuday @ketan @GaneshSPatil @bdpiparva @maheshp @varshavaradarajan @jyotisingh Thoughts?