add latest stable links to WIS2 standards on schemas.wmo.int

[tomkralidis]

Update schemas.wmo.int to add latest links foreach WIS2 standard, such that documentataion can safely point to the latest stable version of a specification without having to change versions.

This means that we can have the following links:

• https://schemas.wmo.int/wcmp/latest
• https://schemas.wmo.int/wnm/latest
• https://schemas.wmo.int/wth/latest

cc @6a6d74 @golfvert @amilan17

[amilan17]

not an amendment, just a task ...