[MLIR][NFC] add doc cross links from/to std.alloca

 Add doc cross links between std.alloca and AutomaticAllocationScope.

Subscribers: mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, grosul1, frgossen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77956