feat: add truncate filter to summary tag (#6105)

Co-authored-by: Till Schweneker <till.schweneker@meltingelements.com>
Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

website/content/docs/beta-features.md

@@ -556,14 +556,15 @@ collections:
   - name: 'posts'
     label: 'Posts'
     folder: '_posts'
-    summary: "{{title | upper}} - {{date | date('YYYY-MM-DD')}}"
+    summary: "{{title | upper}} - {{date | date('YYYY-MM-DD')}} – {{body | truncate(20, '***')}}"
     fields:
       - { label: 'Title', name: 'title', widget: 'string' }
       - { label: 'Publish Date', name: 'date', widget: 'datetime' }
+      - { label: 'Body', name: 'body', widget: 'markdown' }
 ```
 
 The above config will transform the title field to uppercase and format the date field using `YYYY-MM-DD` format.
-Available transformations are `upper`, `lower`, `date('<format>'), default('defaultValue') and ternary('valueForTrue','valueForFalse')`
+Available transformations are `upper`, `lower`, `date('<format>')`, `default('defaultValue')`, `ternary('valueForTrue','valueForFalse')` and `truncate(<number>)`/`truncate(<number>, '<string>')`  
 
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