update dereferencing of conformance URIs #46

[tomkralidis]

As discussed with @6a6d74 @golfvert @efucile, update the below specification URIs to deference as follows:

Conformance	URI	URL to resolve to
WCMP2	http://wis.wmo.int/spec/wcmp/2/conf/core	https://wmo-im.github.io/wcmp2/standard/wcmp2-STABLE.html#_conformance_class_core
WNM	http://wis.wmo.int/spec/wnm/1/conf/core	https://wmo-im.github.io/wis2-notification-message/standard/wis2-notification-message-DRAFT.html#_conformance_class_core
WTH	http://wis.wmo.int/spec/wth/1/conf/core	https://wmo-im.github.io/wis2-topic-hierarchy/standard/wis2-topic-hierarchy-STABLE.html#_conformance_class_core

cc @amilan17