Read the text of a Document board (a board of type "document": one rich-text page people write, for example how a process works). Returns the text as markdown and as the TipTap document the editor stores, plus its revision number and who saved it when. Before anyone has saved, revision, content, markdown, savedAt and savedBy are null. To change the text, send it to saveBoardDocument with this revision as baseRevision. The markdown may simplify tables and collapsible sections; edit through content when the document has those. Requires view access to the board.