fix(v2): add accessible label for Algolia search button

[hobadams]

Motivation

The Algolia search button doesn't contain text which fails lighthouse a11y tests + isn't nice for screen readers.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

This has just been tested manually by checking the markup and running lighthouse.

After

Before

Markup

Related PRs

N/A

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[hobadams]

@yangshun - I've signed the contributor agreement. Are you able to re run that check?

[slorber]

Thanks

@francoischalifour isn't it something you would like to fix in DocSearchButton directly?

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[yangshun]

Thank you!

[slorber]

🤪 😅

so @francoischalifour if it was not the right fix, you can remove it

[francoischalifour]

Thank you @hobadams.

@slorber I included this fix in the next version of DocSearch, I'll make sure to remove this line when I'll bump the DocSearch version on Docusaurus.