Enhance create page blocks doc (#2514)

* create_custom_page_block.md: Highlight that new block must be enabled in BO
* data_migration_actions.md: Add `add_block_to_available_blocks` example

---------

Co-authored-by: Thorsten Reiter <34274688+reithor@users.noreply.github.com>
Co-authored-by: Marek Nocoń <mnocon@users.noreply.github.com>

docs/content_management/data_migration/data_migration_actions.md

@@ -58,6 +58,7 @@ The following migration actions are available out of the box:
 - `remove_drafts` (Content type Update)
 - `remove_field_by_identifier` (Content type Update)
 - `unassign_content_type_group` (Content type Update)
+- `add_block_to_available_blocks` (Content type Update)
 - `assign_role_to_user` (Role Create / Update)
 - `assign_role_to_user_group` (Role Create / Update)
 - `assign_user_to_role` (User Create / Update)
@@ -103,6 +104,7 @@ mode: Update
         - { action: unassign_content_type_group, value: 'Content' }
         - { action: remove_field_by_identifier, value: 'short_title' }
         - { action: remove_drafts, value: null }
+        - { action: add_block_to_available_blocks, fieldDefinitionIdentifier: 'page', blocks: ['event'] }
 ```
 
 ### Roles

docs/content_management/pages/create_custom_page_block.md

@@ -132,5 +132,8 @@ The following UDW configuration is used so only an `event` typed content item ca
 
 For more information, see [UDW configuration](browser.md#udw-configuration).
 
-Your custom page block is now ready. 
-Before you can use it in Page Builder, you must [enable it in Page field settings]([[= user_doc =]]/content_management/configure_ct_field_settings/#block-display).
+Your custom page block is now registered in the system.
+
+!!! caution
+
+   To use the new block in Page Builder, add it to the list of available blocks in a given content type's settings. This can be done manually in [Page field settings]([[= user_doc =]]/content_management/configure_ct_field_settings/#block-display) or by using the migration action [`add_block_to_available_blocks`](data_migration_actions.md#content-types).