add HTML5 validation of docs pages to the build
add ARIA roles to Dropdown examples in Components docs
add src attribute to holder.js-powered <img>s so that they validate

use `data:` URL of empty invalid PNG: src="data:image/png;base64,"
add alt text to <img>s in components.html
add alt text to <img>s in javascript.html
Components docs: Dropdowns section: add .sr-only toggle buttons so aria-labelledby validates
Merge pull request #9396 from twbs/docs-html-validation

add HTML5 validation of docs pages to the build