Merge pull request #554 from humanmade/backport-553-to-v17-branch

[Backport v17-branch] Update documentation to include details of HM-GTM

other-docs/guides/upgrading/v17.md

@@ -41,7 +41,9 @@ composer server cli -- altis migrate
 
 ## Changes to the Altis platform
 
-We have removed the Analytics module, Experience Blocks and Global Blocks from
+### Analytics module
+
+We have removed the Analytics module, which includes Experience Blocks and Global Blocks from
 the Altis platform. These are now delivered through the new Altis Accelerate
 plugin. Learn more at https://www.altis-dxp.com/accelerate/.
 
@@ -75,6 +77,19 @@ module to your `composer.json` file. It is no longer enabled by default.
 }
 ```
 
+### Google Tag Manager
+
+The Google Tag Manager module is also removed as part of this release. If you add `altis/Analytics` back into your
+project, as above, you will also get the Google Tag Manager module. If, instead, you install the Altis Accelerate
+plugin, you will need to manually add the HM Google Tag Manager module to your `composer.json` file.
+
+```shell
+# Add Altis Analytics support
+composer require humanmade/hm-gtm
+```
+
+Once added, you will need to network enable the plugin.
+
 ### PHP 8.1
 
 PHP 8.1 is now our recommended version of PHP for Altis. We have improved