Add related column clarification to VirtualDataset and Grid docs

[Copilot]

VirtualDataset works with savedqueries — previous assumption was wrong. Added clarification on how to work with related (linked entity) columns via dotted notation in the proper places.

• VirtualDataset general.md: Added "Related (Linked Entity) Columns" subsection under FetchXml Provider explaining the {link-entity-alias}.{attribute-name} dotted notation required for columns from link-entity joins. Updated examples to use talxis_ prefix.
• Grid grid.md: Added a note in Column Configuration pointing to the VirtualDataset related columns documentation for views with linked entities.

Example of the dotted notation for a related column in the Columns binding:

[
  ...
  {
    "name": "a_e1371f3c98fd4fa094ca25fbf67eacb0.talxis_description",
    "alias": "a_e1371f3c98fd4fa094ca25fbf67eacb0.talxis_description",
    "dataType": "SingleLine.Text",
    "displayName": "Category Description"
  }
]

The alias (e.g. a_e1371f3c98fd4fa094ca25fbf67eacb0) is auto-generated by Dataverse when adding related columns to a view and must match exactly.

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