docs: climate services overview

[turban]

Summary

• Adds docs/climate_services.md explaining what climate services are (GFCS framework, the production-to-decision chain) and how the Open Climate Service relates to that concept
• Places the Open Climate Service in the climate services information system layer — between global data producers and user-facing tools
• Clarifies the relationship to DHIS2: shared infrastructure for the climate and health ecosystem, operable independently
• Added as the first entry under Concepts in the nav