feat: added Slovenian Translation (#2733)

# Description

Added Slovenian Translation by instructions for adding new translations.

## Checklist

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have performed a self-review of my own code
- [X] I have attached images of the change if it is UI based
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] If my code has heavily changed functionality I have updated
relevant docs on [Stirling-PDFs doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
- [x] My changes generate no new warnings
- [x] 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)

---------

Co-authored-by: blaz.carli <blaz.carli@arctur.si>

src/main/resources/templates/fragments/languages.html

@@ -23,6 +23,7 @@
                       <a class="dropdown-item lang_dropdown-item" href="" data-bs-language-code="pt_PT"> <img th:src="@{'/images/flags/pt_pt.svg'}" alt="icon" width="20" height="15"> Português (PT)</a>
                       <a class="dropdown-item lang_dropdown-item" href="" data-bs-language-code="ro_RO"> <img th:src="@{'/images/flags/ro.svg'}" alt="icon" width="20" height="15"> Romanian</a>
                       <a class="dropdown-item lang_dropdown-item" href="" data-bs-language-code="sk_SK"> <img th:src="@{'/images/flags/sk.svg'}" alt="icon" width="20" height="15"> Slovensky</a>
+                      <a class="dropdown-item lang_dropdown-item" href="" data-bs-language-code="sl_SI"> <img th:src="@{'/images/flags/si.svg'}" alt="icon" width="20" height="15"> Slovenian</a>
                       <a class="dropdown-item lang_dropdown-item" href="" data-bs-language-code="sv_SE"> <img th:src="@{'/images/flags/se.svg'}" alt="icon" width="20" height="15"> Svenska</a>
                       <a class="dropdown-item lang_dropdown-item" href="" data-bs-language-code="tr_TR"> <img th:src="@{'/images/flags/tr.svg'}" alt="icon" width="20" height="15"> Türkçe</a>
                       <a class="dropdown-item lang_dropdown-item" href="" data-bs-language-code="ru_RU"> <img th:src="@{'/images/flags/ru.svg'}" alt="icon" width="20" height="15"> Русский</a>