Add to default port allocations wiki page

[mattbostock]

It'd be good to add an entry to the Prometheus wiki to make alertmanager2es easier to find.

The default port number we're using is 9097, though that doesn't strictly fit under 'Core components'. Perhaps @brian-brazil or @discordianfish could suggest the right place?

[brian-brazil]

Non-exporter stuff has been taking ports from the exporter list, so 9239 is next.