fix many broken links and outdated posts/sections
Created by: sqs
Helps with https://github.com/sourcegraph/sourcegraph/issues/1515.
Updates all links to point to https://docs.sourcegraph.com (instead of relying on redirects). Fixes many other things, such as:
- Uses new terminology consistently (Sourcegraph, not Sourcegraph Server or Sourcegraph Data Center)
- Links to https://docs.sourcegraph.com/integration/browser_extension instead of the Chrome and Firefox stores separately
- References to old features that are now configured differently or that have been removed