discovery.xml: Remove "recent submissions" from homepage Discovery config

The "homepage" Discovery config is meant to be used by the initial
front page of DSpace XMLUI, and we don't want to show those there!

Signed-off-by: Alan Orth <alan.orth@gmail.com>
discovery.xml: Switch site homepage to use the "homepage" Discovery configuration

This makes the homepage behave like XMLUI on DSpace 3.x, where the
recent submissions are not shown.

Signed-off-by: Alan Orth <alan.orth@gmail.com>
discovery.xml: Reduce number of recent submissions shown from 20 -> 5

DSpace 3.x's XMLUI only shows 5, so let's mimic that behavior.

Signed-off-by: Alan Orth <alan.orth@gmail.com>