docs: add mockup generator to README, delete sample-content.md

- Add "Generating Mockups" section documenting mix generate_mockups
- Remove stale reference to deleted docs/spec.md
- Delete docs/plans/sample-content.md (plan fully implemented)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -112,10 +112,37 @@ assets/css/
 | `/dev/errors/404` | Preview 404 page (dev only) |
 | `/dev/errors/500` | Preview 500 page (dev only) |
 
+## Generating Mockups
+
+The project includes a Printify integration for generating product mockups. This is useful for creating sample product images from Unsplash artwork.
+
+### Prerequisites
+
+1. A Printify account with API access
+2. Set the `PRINTIFY_API_TOKEN` environment variable
+
+### Usage
+
+```bash
+# Generate all mockups (saves to priv/static/mockups/)
+export PRINTIFY_API_TOKEN="your-token"
+mix generate_mockups
+
+# Generate mockups and delete products from Printify afterwards
+mix generate_mockups --cleanup
+
+# Search for available blueprints
+mix generate_mockups --search "poster"
+
+# List all blueprints
+mix generate_mockups --list-blueprints
+```
+
+Product definitions are in `lib/simpleshop_theme/printify/mockup_generator.ex`.
+
 ## Documentation
 
 - [ROADMAP.md](ROADMAP.md) - Future features and improvements
-- [docs/spec.md](docs/spec.md) - Theme system specification
 - [docs/plans/](docs/plans/) - Feature plans
 - [docs/research/](docs/research/) - Design research and guidelines