fix(site): make YouTube demo thumbnail clickable

[GITHUBTANMAY2003]

Description

Makes the YouTube demo thumbnail on the dynavec landing page open the correct video in a new tab.

The static facade is converted into a semantic link while preserving its existing appearance and improving keyboard accessibility.

Related issue

Fixes #260

Changes

• Converted the YouTube facade from a static div and nested button into a semantic anchor.
• Added the correct YouTube video URL with safe new-tab attributes.
• Replaced the nested button with a decorative play-icon span.
• Removed obsolete iframe-specific CSS.

Testing

• Manually verified that clicking the thumbnail opens the correct video in a new tab.
• Verified keyboard navigation and Enter-key activation.
• Ruff checks pass.
• Mypy passes.
• Semantic Kernel tests pass separately (8 passed).
• Remaining test suite passes with two optional tests skipped.
• Documentation updated, if applicable (not required).

The combined local Pytest run encountered a pre-existing optional-dependency import-order conflict between DSPy and Semantic Kernel; both test groups pass when run separately.

Checklist

• My changes are focused and relevant to this pull request.
• I have added or updated tests where appropriate.
• I have reviewed my changes for unrelated modifications.
• I have updated documentation where necessary.

Coordination note

Raised at @tikhandesanket’s request because PR #265 also modifies the same landing-page section. This focused PR isolates the issue #260 implementation so the overlapping changes can be reviewed and reconciled.