Update architecture/api_design.inc.rst to add in Summary Statistics domain and namespace

[gajanan-choudhary]

The oneapi::mkl::stats namespace is missing in architecture/api_design.inc.rst (built website link). Please update it to add in the namespace and domain description.

[mkrainiuk]

Assigned to Andrey, for some reasons I can't assign it to @paveldyakov as component owner in uxlfoundation space

[andreyfe1]

Fixed in #587