Sorted Linked List Added (#5519)

* Sorted Linked List added with Javadoc and tests

* "Added comments to SortedLinkedList.java to describe the implementation and provide a reference link."

* Upgraded test from junit 4 to junit 5

* Added space before braces of functions

* Rename SortedlinkedListTest.java to SortedLinkedListTest.java

* made to string null safe

* Updated tail

* "Added assignment of `this.tail` to `newNode` in `SortedLinkedList` class."

* Remove assertions for minValue and maxValue in empty list test

* tried to get link updated

* "Fixed whitespace and formatting issues in SortedLinkedList.java and SortedLinkedListTest.java"

* formatting of test file corrected

* Removed few whitespaces

* Addressed comments by alxkm

* "Updated toString method to include brackets and removed default Node constructor."

* tests updated