fix(core): ExplorerDocNode no longer displays links multiple times

[Demohstens]

Fixes #8522 by creating a new memoized and filtered array of unique linked docs and iterating over the uniqueChildren when creating the Linked ExplorerDocNodes.

Potential issue with Speed: Using a Set might be faster. Especially with large lists of links.

It might be more "elegant" to directly edit the children coming instead of hooking into the changes to children. But I am not very comfortable with the LiveData class so I'm hesitant to use it here. The solution below does work, though.

Version within the children assignment

  const children = useLiveData(
    useMemo(
      () => {
        const data = LiveData.from(
          !collapsed ? docsSearchService.watchRefsFrom(docId) : NEVER,
          null
        )
        return data.map(v => v?.filter(
          (child, index, self) =>
            index === self.findIndex(c => c.docId === child.docId)
          
        )) 
       }, [docsSearchService, docId, collapsed]
    )
  );

[graphite-app]

How to use the Graphite Merge Queue

Add either label to this PR to merge it via the merge queue:

• merge - adds this PR to the back of the merge queue
• hotfix - for urgent hot fixes, skip the queue and merge this PR next

You must have a Graphite account in order to use the merge queue. Sign up using this link.

An organization admin has enabled the Graphite Merge Queue in this repository.

Please do not merge from GitHub as this will restart CI on PRs being processed by the merge queue.