A11y front page labels

[julianna-ciq]

Documentation

(Both on the front page)

• Added an aria-label to the Velocity range slider
• Added descriptions for every example's thumbnail, and surfaced them as alt text.

Validation

I am presently unable to build Perspective in order to validate that this documentation works correctly. I have a discussion open to try to get myself to a point where I can build things.

However, I wanted to submit this PR ahead of time, in case anyone wants to review or change the image descriptions that I wrote.

• I have linted, spell-checked, and grammar-checked my documentation additions.
• I have run yarn docs to regenerate the API documentation from source.
• Within /docs, I have run yarn start to regenerate the Docusaurus site, and I have validated the changes
• My additions are styled and structured correctly on the Docusaurus site

Checklist

• I have read the CONTRIBUTING.md and followed its Guidelines

[finos-cla-bot]

Thank you for your contribution and Welcome to our Open Source Community!

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cc @TheJuanAndOnly99 @mcleo-d

[texodus]

Thanks for the PR @julianna-ciq ! Looks good!