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.

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## 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.