added supporting youtube video links for flexbox and grid

[Punosie]

Because

Students often benefit from visual and audio explanations alongside text content. Adding curated video resources will help reinforce the concepts of Flexbox and Grid through different learning mediums.

This PR

• Added beginner-friendly YouTube video link explaining CSS Flexbox fundamentals
• Added concise YouTube video tutorial demonstrating CSS Grid basics
• Both videos serve as supplementary resources to the existing lesson content

Issue

Closes #29064

Additional Information

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

[rlmoser99]

@Punosie I just realized you created a PR and an issue at the same time. In the future, you should make the issue and wait to hear back from maintainers before you create a PR. If you are just making a small change (like typo/grammar), you can skip the issue and just make the PR.

Since you've made the PR, please see the note that I made on your issue and add more context to this PR.

[Punosie]

Got it