Accessibility Handbook: add Speech Synthesizer

[asiciliano]

Summary

Add Speech Synthesizer to the Accessibility Handbook. It is useful for blind users who use a screen reader.

[pauamma_gundo.com]

documentation/content/en/books/accessibility/blindness/_index.adoc

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Also link to https://docs.freebsd.org/en/books/handbook/multimedia/?

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[asiciliano]

documentation/content/en/books/accessibility/blindness/_index.adoc

167

I' ll add the link.

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I'll fix.

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I'll fix.

436

I'll fix.

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I'll fix.

[asiciliano]

Thank you for the review. I updated the diff to fix

[bcr]

Looks good to me, thank you!

[jrm]

@asiciliano, based on the review from @pauamma_gundo.com and @bcr, this is ready to be pushed whenever you are ready.