refactor: move modules under app/ directory and update file paths (#3938)

# Description of Changes

- **What was changed:**  
- Renamed top-level directories: `stirling-pdf` → `app/core`, `common` →
`app/common`, `proprietary` → `app/proprietary`.
- Updated all path references in `.gitattributes`, GitHub workflows
(`.github/workflows/*`), scripts (`.github/scripts/*`), `.gitignore`,
Dockerfiles, license files, and template settings to reflect the new
structure.
- Added a new CI job `check-generateOpenApiDocs` to generate and upload
OpenAPI documentation.
- Removed redundant `@Autowired` annotations from `TempFileShutdownHook`
and `UnlockPDFFormsController`.
- Minor formatting and comment adjustments in YAML templates and
resource files.

- **Why the change was made:**  
- To introduce a clear `app/` directory hierarchy for core, common, and
proprietary modules, improving organization and maintainability.
- To ensure continuous integration and Docker builds continue to work
seamlessly with the reorganized structure.
- To automate OpenAPI documentation generation as part of the CI
pipeline.

---

## 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)
- [x] 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.

devGuide/DeveloperGuide.md

@@ -332,7 +332,7 @@ Thymeleaf is a server-side Java HTML template engine. It is used in Stirling-PDF
 
 ### Thymeleaf overview
 
-In Stirling-PDF, Thymeleaf is used to create HTML templates that are rendered on the server side. These templates are located in the `stirling-pdf/src/main/resources/templates` directory. Thymeleaf templates use a combination of HTML and special Thymeleaf attributes to dynamically generate content.
+In Stirling-PDF, Thymeleaf is used to create HTML templates that are rendered on the server side. These templates are located in the `app/core/src/main/resources/templates` directory. Thymeleaf templates use a combination of HTML and special Thymeleaf attributes to dynamically generate content.
 
 Some examples of this are:
 
@@ -384,7 +384,7 @@ This would generate n entries of tr for each person in exampleData
 ### Adding a New Feature to the Backend (API)
 
 1. **Create a New Controller:**
-   - Create a new Java class in the `stirling-pdf/src/main/java/stirling/software/SPDF/controller/api` directory.
+   - Create a new Java class in the `app/core/src/main/java/stirling/software/SPDF/controller/api` directory.
    - Annotate the class with `@RestController` and `@RequestMapping` to define the API endpoint.
    - Ensure to add API documentation annotations like `@Tag(name = "General", description = "General APIs")` and `@Operation(summary = "Crops a PDF document", description = "This operation takes an input PDF file and crops it according to the given coordinates. Input:PDF Output:PDF Type:SISO")`.
 
@@ -411,7 +411,7 @@ This would generate n entries of tr for each person in exampleData
    ```
 
 2. **Define the Service Layer:** (Not required but often useful)
-   - Create a new service class in the `stirling-pdf/src/main/java/stirling/software/SPDF/service` directory.
+   - Create a new service class in the `app/core/src/main/java/stirling/software/SPDF/service` directory.
    - Implement the business logic for the new feature.
 
    ```java
@@ -463,7 +463,7 @@ This would generate n entries of tr for each person in exampleData
 ### Adding a New Feature to the Frontend (UI)
 
 1. **Create a New Thymeleaf Template:**
-   - Create a new HTML file in the `stirling-pdf/src/main/resources/templates` directory.
+   - Create a new HTML file in the `app/core/src/main/resources/templates` directory.
    - Use Thymeleaf attributes to dynamically generate content.
    - Use `extract-page.html` as a base example for the HTML template, which is useful to ensure importing of the general layout, navbar, and footer.
 
@@ -507,7 +507,7 @@ This would generate n entries of tr for each person in exampleData
    ```
 
 2. **Create a New Controller for the UI:**
-   - Create a new Java class in the `stirling-pdf/src/main/java/stirling/software/SPDF/controller/ui` directory.
+   - Create a new Java class in the `app/core/src/main/java/stirling/software/SPDF/controller/ui` directory.
    - Annotate the class with `@Controller` and `@RequestMapping` to define the UI endpoint.
 
    ```java
@@ -537,7 +537,7 @@ This would generate n entries of tr for each person in exampleData
 
 3. **Update the Navigation Bar:**
    - Add a link to the new feature page in the navigation bar.
-   - Update the `stirling-pdf/src/main/resources/templates/fragments/navbar.html` file.
+   - Update the `app/core/src/main/resources/templates/fragments/navbar.html` file.
 
    ```html
    <li class="nav-item">
@@ -551,7 +551,7 @@ When adding a new feature or modifying existing ones in Stirling-PDF, you'll nee
 
 ### 1. Locate Existing Language Files
 
-Find the existing `messages.properties` files in the `stirling-pdf/src/main/resources` directory. You'll see files like:
+Find the existing `messages.properties` files in the `app/core/src/main/resources` directory. You'll see files like:
 
 - `messages.properties` (default, usually English)
 - `messages_en_GB.properties`