ci: fix Swagger docs generation by targeting stirling-pdf module (#3935)

# Description of Changes

**What was changed**

- Updated the GitHub Actions workflow (`.github/workflows/swagger.yml`)
to invoke the `:stirling-pdf:generateOpenApiDocs` task instead of the
root `generateOpenApiDocs`. Refactored `build.gradle` to apply the
`org.springdoc.openapi-gradle-plugin` exclusively to the `stirling-pdf`
subproject, configured its `openApi` extension, and introduced new
Gradle tasks—`copySwaggerDoc` and `cleanSwaggerInBuild`—to manage the
generated `SwaggerDoc.json` file correctly.

**Why the change was made**

- The previous configuration failed to generate OpenAPI documentation
for the `stirling-pdf` module. These changes ensure that Swagger
documentation is produced from the correct module, uploaded to
SwaggerHub as intended, and that temporary artifacts are cleaned up to
maintain a tidy build directory.


try #3932

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

.github/workflows/swagger.yml

@@ -29,7 +29,7 @@ jobs:
       - uses: gradle/actions/setup-gradle@ac638b010cf58a27ee6c972d7336334ccaf61c96 # v4.4.1
 
       - name: Generate Swagger documentation
-        run: ./gradlew generateOpenApiDocs
+        run: ./gradlew :stirling-pdf:generateOpenApiDocs
 
       - name: Upload Swagger Documentation to SwaggerHub
         run: ./gradlew swaggerhubUpload