Add visibly hidden button text for screen reader users.

[Finishes #10448]
fix swapped angle brackets in docs; fixes part of #10456
add <code> around ".less" in Getting Started docs
Merge pull request #10453 from ckundo/add-dropdown-button-text

Add visibly hidden button text for screen reader users.
[doc] fixed the horizontal form example
Merge pull request #10471 from javiereguiluz/patch-4

[doc] fixed the horizontal form example