♻️(backend) remove content from list serializer and introduce excerpt

Including the content field in the list view is not efficient as we need
to query the object storage to retrieve it. We want to display an excerpt
of the content on the list view so we should store it in database. We
let the frontend compute it and save it for us in the new "excerpt" field
because we are not supposed to have access to the content (E2EE feature coming)

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to
 ## Added
 
 - ✨(backend) allow organizing documents in a tree structure #516
+- ✨(backend) add "excerpt" field to document list serializer #516
 - ✨(backend) add github actions to manage Crowdin workflow #559 & #563
 - 📈Integrate Posthog #540
 - 🏷️(backend) add content-type to uploaded files #552
@@ -22,6 +23,10 @@ and this project adheres to
 - 💄(frontend) add abilities on doc row #581
 - 💄(frontend) improve DocsGridItem responsive padding  #582
 
+## Removed
+
+- 🔥(backend) remove "content" field from list serializer # 516
+
 ## [2.0.1] - 2025-01-17
 
 ## Fixed