Add a task-oriented usage guide

docs/GUIDE.md walks through the jobs people actually do, rather than listing
routes: minting a token and fetching a first tile, wiring the tile template into
Leaflet and OpenLayers, accumulating drone imagery across several flights and
deciding when a mosaic rebuild is worth it, integrating a downstream system, a
symptom-to-cause troubleshooting table, and the day-to-day console actions.

API.md stays the reference; the README points at both.

README.md

@@ -115,8 +115,11 @@ Supported token delivery:
 
 ## API
 
-See [docs/API.md](./docs/API.md) for a calling guide with auth, request/response
-examples and the LAN exposure checklist.
+- [docs/GUIDE.md](./docs/GUIDE.md) — task-oriented walkthrough: first tile,
+  integrating with a map frontend, accumulating drone imagery over several
+  flights, downstream integration and troubleshooting.
+- [docs/API.md](./docs/API.md) — endpoint reference: auth, every route grouped by
+  purpose, slicing request/job shapes, error codes, LAN exposure checklist.
 
 ### System