docs: rewrite slide decks with glossary and less jargon

Both decks now open with a plain-language glossary defining
MIR, mono item, allocation, span, terminator, and (in the
reify deck) function item vs. function pointer. The const
deck defines monomorphization, associated constant, and
const generic.

Throughout both decks, compiler-internal event names
(FunctionCallResolved, AllocationCollected, etc.) are
replaced with conversational descriptions ("found a
function call," "found compile-time memory"). The event
names still appear in the trace JSON; the slides just
don't lead with them.

Author: cds-amal