feat(editor): add experimental feature citation (#11984)

Closes: [BS-3122](https://linear.app/affine-design/issue/BS-3122/footnote-definition-adapter-适配)
Closes: [BS-3123](https://linear.app/affine-design/issue/BS-3123/几个-block-card-view-适配-footnote-态)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **New Features**
  - Introduced a new citation card component and web element for displaying citations.
  - Added support for citation-style rendering in attachment, bookmark, and linked document blocks.
  - Enabled citation parsing from footnote definitions in markdown for attachments, bookmarks, and linked docs.
  - Added a feature flag to enable or disable citation features.
  - Provided new toolbar logic to disable downloads for citation-style attachments.

- **Improvements**
  - Updated block models and properties to support citation identifiers.
  - Added localization and settings for the citation experimental feature.
  - Enhanced markdown adapters to recognize and process citation footnotes.
  - Included new constants and styles for citation card display.

- **Bug Fixes**
  - Ensured readonly state is respected in block interactions and rendering for citation blocks.

- **Documentation**
  - Added exports and effects for new citation components and features.

- **Tests**
  - Updated snapshots to include citation-related properties in block data.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/affine/blocks/note/src/adapters/markdown.ts

@@ -3,14 +3,13 @@ import {
   BlockMarkdownAdapterExtension,
   type BlockMarkdownAdapterMatcher,
   FOOTNOTE_DEFINITION_PREFIX,
+  isFootnoteDefinitionNode,
   type MarkdownAST,
 } from '@blocksuite/affine-shared/adapters';
-import type { FootnoteDefinition, Root } from 'mdast';
+import { FeatureFlagService } from '@blocksuite/affine-shared/services';
+import type { Root } from 'mdast';
 
 const isRootNode = (node: MarkdownAST): node is Root => node.type === 'root';
-const isFootnoteDefinitionNode = (
-  node: MarkdownAST
-): node is FootnoteDefinition => node.type === 'footnoteDefinition';
 
 const createFootnoteDefinition = (
   identifier: string,
@@ -67,10 +66,35 @@ const createNoteBlockMarkdownAdapterMatcher = (
         }
       });
 
-      // Remove the footnoteDefinition node from the noteAst
-      noteAst.children = noteAst.children.filter(
-        child => !isFootnoteDefinitionNode(child)
-      );
+      const { provider } = context;
+      let enableCitation = false;
+      try {
+        const featureFlagService = provider?.get(FeatureFlagService);
+        enableCitation = !!featureFlagService?.getFlag('enable_citation');
+      } catch {
+        enableCitation = false;
+      }
+      if (enableCitation) {
+        // if there are footnoteDefinition nodes, add a heading node to the noteAst before the first footnoteDefinition node
+        const footnoteDefinitionIndex = noteAst.children.findIndex(child =>
+          isFootnoteDefinitionNode(child)
+        );
+        if (footnoteDefinitionIndex !== -1) {
+          noteAst.children.splice(footnoteDefinitionIndex, 0, {
+            type: 'heading',
+            depth: 6,
+            data: {
+              collapsed: true,
+            },
+            children: [{ type: 'text', value: 'Sources' }],
+          });
+        }
+      } else {
+        // Remove the footnoteDefinition node from the noteAst
+        noteAst.children = noteAst.children.filter(
+          child => !isFootnoteDefinitionNode(child)
+        );
+      }
     },
   },
   fromBlockSnapshot: {