Update documentation to say IPM should not be installed directly in %SYS and mapped elsewhere

Upgrading systems with IPM installed in %SYS and mapped everywhere else is tricky because the %-classes get wiped out. (See https://docs.intersystems.com/iris20241/csp/docbook/DocBook.UI.Page.cls?KEY=GCI_post). We should update the docs (README and/or wiki) to mention this and recommend installing IPM elsewhere.