Replace raw iframe with docs-video component to fix YouTube embed overflowing on mobile viewports.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/contributing-docs/commit-message-guidelines.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.dev application / infrastructure changes
• Other... Please describe:

What is the current behavior?

The v21 event page embeds the YouTube developer event video using a raw <iframe> with a fixed width="560", causing the video to overflow and get clipped on mobile viewports.

Issue Number: N/A

What is the new behavior?

Uses the <docs-video> component, which applies width: 100% and aspect-ratio: 16/9 , making the embed responsive and consistent with every other YouTube video across the docs.

Does this PR introduce a breaking change?

• Yes
• No

Other information