Update UDF string memory-management guide for the MLIR/NRT backend

Rewrite docs/cudf/source/cudf/developer_guide/udf_memory_management.md to
describe the pure-MLIR string UDF backend: an mlir_string is a flat
{meminfo, data, nbytes} struct allocated via NRT (NRT_MemInfo_new_varsize),
with no C++ shim layer or cudf::udf_string class. Keeps the NRT concepts and
the str1 + str2 lifecycle walkthrough, retargets the data structures,
creation (lower_concat / allocate_mlir_string) and teardown
(free_mlir_string_array / _mlir_string_array_to_column) to the current code,
and adds sections on borrowed-vs-owned strings, enabling NRT, and nullable
(Masked) strings.

Author: brandon-b-miller