update doc content for critical/site/external-service configs
Created by: sqs
fix https://github.com/sourcegraph/sourcegraph/issues/2067
- distinguishes between critical and site config correctly (and external service config)
- documents each external service individually
- uses JSON Schema doc generation from https://github.com/sourcegraph/docsite/pull/9 so that the JSON Schema docs are up to date and do not require manual updating
For the reviewer:
There is a HUGE component of this PR that is fixing incorrect documentation (that does not properly distinguish between site/critical/external-service configuration). This causes confusion (eg https://github.com/sourcegraph/sourcegraph/issues/2251). To fix these inaccuracies, it took a lot of reorganization. Please be explicit when you review if you want to DELAY MERGING this (and DELAY fixing the inaccuracies). I'll assume that your review comments can be addressed in post-merge review unless you explicitly say you want to delay merging this.