Add section on WCMP2 KPIs

[amilan17]

https://github.com/wmo-im/wcmp2/tree/main/kpi
https://wmo-im.github.io/wcmp2/kpi/wcmp2-kpi-DRAFT.html

[6a6d74]

Amend the Guide to add a section on the WCMP2 KPIs and include cross-references where they are mentioned elsewhere in the Guide.

[6a6d74]

@tomkralidis - one for ET-Metadata? If so, can you add a label and select an appropriate milestone? Thanks.

[tomkralidis]

@6a6d74 I've added a FT2026-1 milestone, and we'll push this forward in TT-WISDMD.