New simpler readme, removed outdated docs

Now that the updated Gatewayd documentation is available on the dev portal, we should remove the older, less-detailed, redundant documentation from this repo and link to the preferred docs.

This pull request slims the README.md file down to just an intro, dependencies, installation & updating -- all of them matching the updated Ripple Dev Portal versions -- and adds a link to the Gatewayd documentation in the Dev Portal. It also removes a few documentation files that were out of date and redundant with the Dev Portal docs.