(#423) Updates nci logo in header

[dlescarbeau]

• removes aria label from anchor

• adds alt text value to image

Closes #423

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[github-actions]

Viewing Information

• The documentation site
• NCIDS JS Documentation
• The storybook site
• The example site

[andyvanavery31]

@dlescarbeau could you please update the alt-text in the actual Doc Site header (not the component page) to be "National Cancer Institute Design System Homepage"?

[andyvanavery31]

Passes Product review.

[olitharp-nci]

The header should be using the twig but I'd like to avoid scope creep for this PR. We should make another ticket, but otherwise this looks good