pipe characters in cell content need to be escaped for markdown export

if there's a pipe character in a cell it will bork the column counting of GFM markdown. As such we need to escape them

Locate the markdown export implementation in the repository and trace how cell content is rendered into GFM rows. Reproduce a table containing a pipe character, then verify that the exported markdown preserves the intended column structure after the escaping change.

Written by the indexing model from the issue text.