singly_linked_list: Added additional documentation, type hints and test cases (#4988)

This is a followup to https://github.com/TheAlgorithms/Python/pull/4973#issuecomment-933117382

As per given suggestion, I've added type hints to certain methods that don't have them.  I have also added documentation and example doctests as a usage example for (most of) those that don't have them.

I have also added another test case following the previous test case's format. I noticed that the existing test case from previous pull request might be redundant with the ones I've made, so I decided to create a specific situation where the linked list would have to keep different kinds of data types for each node, in `test_singly_linked_list_2` test function.

Some minor changes in strings has been done to keep things consistent with other parts of the document. If it is undesirable, please let me know.