docs: Add explanation of shunting in process model

Solves issue- #837

The current documentation present in architecture.rst explains the flow of message handling in Mailman but does not describe how errors during message processing are handled. The documentation also references the unshunt command but never explained shunting.

This changes adds an explanation of message shunting in process model section in architecture.rst. The new section describes when messages are moved to the shunt queue, the types of errors that can cause this, and how it differs from other message dispositions such as discard, reject, and hold.

I'll open a follow up MR in mailman-suite-docs repository to add a FAQ entry about shunting that links to this architecture section.