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.

HowToAddNewLanguage.md

@@ -10,7 +10,7 @@ Fork Stirling-PDF and create a new branch out of `main`.
 
 Then add a reference to the language in the navbar by adding a new language entry to the dropdown:
 
-- Edit the file: [languages.html](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/src/main/resources/templates/fragments/languages.html)
+- Edit the file: [languages.html](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/stirling-pdf/src/main/resources/templates/fragments/languages.html)
 
 
 For example, to add Polish, you would add:
@@ -25,7 +25,7 @@ The `data-bs-language-code` is the code used to reference the file in the next s
 
 Start by copying the existing English property file:
 
-- [messages_en_GB.properties](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/src/main/resources/messages_en_GB.properties)
+- [messages_en_GB.properties](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/stirling-pdf/src/main/resources/messages_en_GB.properties)
 
 Copy and rename it to `messages_{your data-bs-language-code here}.properties`. In the Polish example, you would set the name to `messages_pl_PL.properties`.