Multi module refactor (#3640)

# Description of Changes

Migrated Stirling PDF to a multi-module structure:

* Introduced new `:stirling-pdf` module
* Moved all the core logic and features of Stirling PDF into
`:stirling-pdf`
* Updated paths of jobs and scripts

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/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/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] 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/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/DeveloperGuide.md#6-testing)
for more details.

stirling-pdf/src/main/resources/static/css/stamp.css

@@ -0,0 +1,41 @@
+.a4container {
+  position: relative;
+  width: 50%;
+  aspect-ratio: 0.707;
+  border: 1px solid #ddd;
+  box-sizing: border-box;
+  background-color: white;
+}
+
+.pageNumber {
+  position: absolute;
+  display: flex;
+  justify-content: center;
+  align-items: center;
+  font-size: 1em;
+  color: #333;
+  cursor: pointer;
+  background-color: #ccc;
+  width: 15%;
+  height: 15%;
+  transform: translate(-50%, -50%);
+}
+
+.pageNumber:hover {
+  background-color: #eee;
+}
+
+#myForm {
+  display: flex;
+  justify-content: center;
+  align-items: center;
+  margin-top: 20px;
+}
+
+.selectedPosition {
+  background-color: #0a0;
+}
+
+.selectedPosition.selectedHovered {
+  background-color: #006600;
+}