Write "row store" subpage for Architecture Guide

Fill out the existing stub page for row storage, in src/docs/arch-row.dox.

Please coordinate with the writer of "column store" subpage: WT-6906. It's possible/likely that columns and rows may share some text. One way to do this could be to have the arch-index page diagram point to a common page arch-row-column.dox that talks about the common parts and then have that have subpages for the row and column specific stuff. Or maybe no subpages are needed, just a single page that talks about both and highlights any differences as they come up. A slight wrinkle is that another specialization of column store is fixed column store. I don't think it's widely used, but it's certainly there.