Add screenreader text to prev/next carousel buttons in examples to improve accessibility

Refs #13556.
Per https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#carousel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin

[skip sauce]
Merge pull request #14072 from twbs/carousel-gloss-buttons

Add screenreader text to prev/next carousel buttons in examples to improve accessibility