[Feature Request] Bulk action "Link" to link the selected documents to each other

[YakGravity]

Description

Add an Actions > Link bulk operation that links the selected documents to each other.

Today, linking related documents in bulk is possible only through Custom fields, which works as
a value assignment: every selected document receives the same value. For a document link field
that value is itself a document, which has to be found by hand in a picker.

That shape does not match the intent. When several documents belong together, there is no natural
"centre" to point at, and picking one arbitrarily produces a star rather than a group.

Concrete case: a supplier answers an order with one email carrying an order confirmation and
two approval drawings. These arrive as three separate documents. They belong together, and none of
them is the obvious centre.

Proposal. Add Link to the bulk Actions menu, next to Merge and Merge as versions:

1. Select the documents that belong together
2. Actions > Link
3. Pick which document link field to use, if more than one exists
4. Confirm

Every selected document then links to every other selected document. No picker, no search: the
selection is the input.

Why it fits there. The Actions menu already holds operations that treat the selection as a
set: merge, merge as versions, rotate. Linking is the same kind of operation. The Edit bar next
to it holds value assignments, which is a different shape, and that is exactly why the current
path feels awkward.

Other

• The dialog already knows the selection, it is shown as documentCount in the title of the bulk
  custom fields dialog. It is simply not offered as the value.
• modify_custom_fields already calls reflect_doclinks, so the symmetrical half is in place.
• Related: the Actions > Link idea was raised in passing in discussion [Feature Request] Merge Documents as Versions Instead of Combining Files #13252 but was never
  filed on its own, since that discussion was closed once "merge as versions" shipped.
• Please decide explicitly whether the action should add to existing links or replace them.
  Bulk value assignment replaces today ([BUG] Bulk-applying value of related documents removes existing relations instead of merging #13745), which was confirmed as intended, but for a
  grouping action adding is likely the expected behaviour.