Reducing State: additional resource

[fadingmorsecode]

Because

This resource would benefit learners who might need another example or introduction to the hook in this format.
The video provides a relatively simple explanation of the hook with an example that is familiar to learners (todo list).

This PR

• Adds an additional resource (video) to help explain the useReducer hook.

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• If any lesson files are included in this PR, they follow the Layout Style Guide