Docs: add Project/Region/Occurrence entity docs

documentation/entities/region.md

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+# Regions
+
+Regions represent a named geographic region that can be associated with:
+
+- one or more countries (`now_reg_coord_country`)
+- one or more coordinators / people (`now_reg_coord_people` → `com_people`)
+
+## Database model
+
+The main region record is `now_reg_coord`:
+
+- `reg_coord_id` (primary key)
+- `region` (name)
+
+Relations:
+
+- `now_reg_coord_country` rows (composite key: `reg_coord_id` + `country`)
+- `now_reg_coord_people` rows (composite key: `reg_coord_id` + `initials`)
+
+## API endpoints
+
+All region endpoints are admin-only:
+
+- `GET /region/all`
+- `GET /region/:id`
+- `PUT /region` (create/update; returns `{ reg_coord_id }`)
+- `DELETE /region/:id`
+