update dereferencing of specification URIs

[tomkralidis]

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

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

cc @amilan17