docs(a11y): add accessibility scan button

[dancormier]

This PR adds an "accessibility scan" button to the Stacks docs. We might want to instead just opt for an extension like the axe DevTools extension.

Btw, this is real rough with just a quick-once over to kinda have this as a proof of concept.

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[allejo]

Just a drive-by thought here, if this PoC goes anywhere, why not bundle it as a separate package within the @StackExchange npm namespace and ship it? That way we're not shipping with axe-tools with Stacks but it's easy enough to pull in and replace the current implementation that exists in some places.

[b-kelly]

@allejo I agree. This PoC was quickly thrown together at the request of an employee internally. I personally think that this is something that would be better suited for a Tampermonkey script