Add max size configuration to Nuxeo Quota docs

Add a Configuration section on the Nuxeo Quote page or a new HOWTO page that explains how to configure the quota max size.

The quota limit used to be hardcoded to 999GB. It is now configurable by contributing an extension like:

<require>org.nuxeo.ecm.quota.maxsize.config</require>
  <extension target="org.nuxeo.runtime.ConfigurationService" point="configuration">
    <property name="nuxeo.quota.maxsize">2 TB</property>
  </extension>

The size is expressed in bytes but can use a suffix like KB, MB, GB or TB.