Revamp `md5.py`

[tianyizheng02]

Describe your change:

Revamped hashes/md5.py:

• Added type hints to all functions
• Added doctests to all functions
• Greatly expanded documentation for all functions
• Added input validation to some functions as needed
• Added reference to the Wikipedia article that the implementation is based on
• Renamed some variables and functions to clarify their usage
• Minor refactoring for clarity

There's a lot in this PR, so let me know if anything else should be changed or if any existing changes should be reverted

• [ ] Add an algorithm?
• [x] Fix a bug or typo in an existing algorithm?
• [x] Documentation change?

Checklist:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [x] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [ ] All new Python files are placed inside an existing directory.
• [ ] All filenames are in all lowercase characters with no spaces or dashes.
• [x] All functions and variable names follow Python naming conventions.
• [x] All function parameters and return values are annotated with Python type hints.
• [x] All functions have doctests that pass the automated testing.
• [x] All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• [ ] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.