Replace Slack API Ref with official OpenAPI Spec

[Roach]

https://github.com/slackapi/slack-api-specs

[jmanian]

Unfortunately the OpenAPI spec is not actively maintained at the moment and has gotten fairly out of date. I think we need to hold on this until the spec is being kept up to date more regularly, or else we won't have access to the newest methods.

I chatted with @episod in the Slack dev hangout and there's no concrete timeline of when it will be resurrected.

[dblock]

Ugh. Sounds familiar. So we continue maintaining a community https://github.com/slack-ruby/slack-api-ref that can be kept up-to-date as needed by the consumers.