Correct links to use gitlab.example.com

Fix links so they don't imply actual GitLab URLs, but instead use
variants of example.com.

doc/administration/reply_by_email_postfix_setup.md

@@ -306,7 +306,7 @@ Courier, which we will install later to add IMAP authentication, requires mailbo
 
       ```shell
       Trying 123.123.123.123...
-      Connected to mail.example.gitlab.com.
+      Connected to mail.gitlab.example.com.
       Escape character is '^]'.
       - OK [CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA IDLE ACL ACL2=UNION] Courier-IMAP ready. Copyright 1998-2011 Double Precision, Inc.  See COPYING for distribution information.
       ```

doc/api/environments.md

@@ -34,7 +34,7 @@ Example response:
     "id": 1,
     "name": "review/fix-foo",
     "slug": "review-fix-foo-dfjre3",
-    "external_url": "https://review-fix-foo-dfjre3.example.gitlab.com",
+    "external_url": "https://review-fix-foo-dfjre3.gitlab.example.com",
     "state": "available"
   }
 ]
@@ -62,7 +62,7 @@ Example of response
   "id": 1,
   "name": "review/fix-foo",
   "slug": "review-fix-foo-dfjre3",
-  "external_url": "https://review-fix-foo-dfjre3.example.gitlab.com",
+  "external_url": "https://review-fix-foo-dfjre3.gitlab.example.com",
   "state": "available",
   "last_deployment": {
     "id": 100,
@@ -164,7 +164,7 @@ POST /projects/:id/environments
 | `external_url` | string  | no     | Place to link to for this environment |
 
 ```shell
-curl --data "name=deploy&external_url=https://deploy.example.gitlab.com" --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/projects/1/environments"
+curl --data "name=deploy&external_url=https://deploy.gitlab.example.com" --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/projects/1/environments"
 ```
 
 Example response:
@@ -174,7 +174,7 @@ Example response:
   "id": 1,
   "name": "deploy",
   "slug": "deploy",
-  "external_url": "https://deploy.example.gitlab.com",
+  "external_url": "https://deploy.gitlab.example.com",
   "state": "available"
 }
 ```
@@ -197,7 +197,7 @@ PUT /projects/:id/environments/:environments_id
 | `external_url`  | string  | no                                | The new `external_url`           |
 
 ```shell
-curl --request PUT --data "name=staging&external_url=https://staging.example.gitlab.com" --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/projects/1/environments/1"
+curl --request PUT --data "name=staging&external_url=https://staging.gitlab.example.com" --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/projects/1/environments/1"
 ```
 
 Example response:
@@ -207,7 +207,7 @@ Example response:
   "id": 1,
   "name": "staging",
   "slug": "staging",
-  "external_url": "https://staging.example.gitlab.com",
+  "external_url": "https://staging.gitlab.example.com",
   "state": "available"
 }
 ```
@@ -253,7 +253,7 @@ Example response:
   "id": 1,
   "name": "deploy",
   "slug": "deploy",
-  "external_url": "https://deploy.example.gitlab.com",
+  "external_url": "https://deploy.gitlab.example.com",
   "state": "stopped"
 }
 ```

doc/topics/offline/quick_start_guide.md

@@ -34,7 +34,7 @@ Follow these steps to enable SSL for your fresh instance. Note that these steps
 
    ```ruby
    # Update external_url from "http" to "https"
-   external_url "https://example.gitlab.com"
+   external_url "https://gitlab.example.com"
 
    # Set Let's Encrypt to false
    letsencrypt['enable'] = false
@@ -64,8 +64,8 @@ Follow these steps to enable the container registry. Note that these steps refle
 
    ```ruby
    # Change external_registry_url to match external_url, but append the port 4567
-   external_url "https://example.gitlab.com"
-   registry_external_url "https://example.gitlab.com:4567"
+   external_url "https://gitlab.example.com"
+   registry_external_url "https://gitlab.example.com:4567"
    ```
 
 1. Reconfigure your instance to apply the changes: