Merge branch 'rdoursenaud/gitlab-ce-omniauth-bitbucket' into 'master'

Update Bitbucket OmniAuth documentation

## What does this MR do?

Updates the Bitbucket integration documentation.

## Why was this MR needed?

Current documentation is out of date and very confusing.

## What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/2137 and https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1052.

See merge request !6106

doc/integration/omniauth.md

@@ -102,8 +102,8 @@ To change these settings:
         block_auto_created_users: true
     ```
 
-Now we can choose one or more of the Supported Providers listed above to continue
-the configuration process.
+Now we can choose one or more of the [Supported Providers](#supported-providers)
+listed above to continue the configuration process.
 
 ## Enable OmniAuth for an Existing User