docs: add introductory presentation for Open Climate Service

[turban]

Self-contained HTML presentation at presentations/introduction/index.html built with Reveal.js.

13 slides:

1. Title
2. The Challenge — fragmented provider landscape
3. What Is a Climate Service? — WMO context, beyond health
4. 2½ Years of Learning — four key lessons (fragment-animated)
5. Stage 1 — Climate App + Google Earth Engine (animated data flow)
6. Stage 2 — Opening the Toolbox / Climate Tools (auto-animated from stage 1)
7. Stage 3 — Open Climate Service (auto-animated from stage 2)
8. Digital Sovereignty — The HISP Way
9. What's Included — data types and integrations
10. The Power of Gridded Data — visual comparison
11. Built on Open Standards — Zarr, GeoZarr, STAC, OGC API, FAIR
12. Roadmap
13. Team + links

Open presentations/introduction/index.html directly in a browser (requires internet for fonts and Reveal.js CDN). Navigate with arrow keys or spacebar.