Added option to split PDF into multiple parts and merge into one PDF (#841)

* Added option to split PDF into multiple parts and merge into one PDF

* Use the mergeDocuments method in MergeController to implement merging

---------

Co-authored-by: Eric <71648843+sbplat@users.noreply.github.com>

src/main/resources/templates/split-pdf-by-sections.html

@@ -23,6 +23,9 @@
                   <label for="verticalDivisions" th:text="#{split-by-sections.vertical.label}">Vertical Divisions</label>
                   <input type="number" id="verticalDivisions" name="verticalDivisions" class="form-control" min="0" max="300" required value="1" th:placeholder="#{split-by-sections.vertical.placeholder}">
                   <br>
+                  <label for="verticalDivisions" th:text="#{split-by-sections.merge}">merge PDFs into one</label>
+                  <input type="checkbox" id="merge" name="merge" th:placeholder="#{split-by-sections.merge}">
+                  <br>
                   <div id="pdfVisualAid" class="pdf-visual-aid"></div>
                   <script>
                     function updateVisualAid() {