Standardize Quoted Path Syntax in Thymeleaf th:href for Consistent Link Resolution (#3515)

# Description of Changes

Please provide a summary of the changes, including:

- **What was changed**  
1. In `DeveloperGuide.md`, updated the example `<a>` tag to use
`th:href="@{'/new-feature'}"` instead of `th:href="@{/new-feature}"`.
2. In `adminSettings.html`, replaced the static `href="/usage"` with a
Thymeleaf attribute `th:href="@{'/usage'}"` to ensure proper URL
resolution.

- **Why the change was made**  
Consistently quoting paths in `th:href` improves readability and avoids
potential parsing inconsistencies in Thymeleaf templates when resolving
URLs at runtime.

---

## 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.

DeveloperGuide.md

@@ -541,7 +541,7 @@ This would generate n entries of tr for each person in exampleData
 
    ```html
    <li class="nav-item">
-       <a class="nav-link" th:href="@{/new-feature}">New Feature</a>
+       <a class="nav-link" th:href="@{'/new-feature'}">New Feature</a>
    </li>
    ```
 

src/main/resources/templates/adminSettings.html

@@ -51,7 +51,7 @@
 			        <span th:text="#{adminUserSettings.changeUserRole}">Change User's Role</span>
 			    </a>
 			    
-			    <a href="/usage" th:if="${@runningEE}"
+			    <a th:href="@{'/usage'}" th:if="${@runningEE}"
 				   class="btn btn-outline-success" 
 				   th:title="#{adminUserSettings.usage}">
 				    <span class="material-symbols-rounded">analytics</span>
@@ -316,4 +316,4 @@
       <th:block th:insert="~{fragments/footer.html :: footer}"></th:block>
     </div>
   </body>
-</html>
+</html>