Write "dhandle/btree" subpage for Architecture Guide

In the top level page, dhandle/btree are in their own box, as btree is a specialization of a dhandle.  It may be appropriate to talk about this, and then have subsections to discuss Btrees and Dhandles separately.  If this seems unnatural, we can have separate pages (and we'll probably have the master pointer from the top level diagram be to btree, though you may have a better idea).

As a special request, in the Btree section, we should cover Truncation. It's an interesting mechanism, and we've chased bugs in it related to prepared transaction support and durable history.