``aria-label="close"`` instead of sr-only ``span``

BS3 commits: c609272ac199cc9806ad6c4cf8c7f8f4253fb347
Make sidebar navigation actual ``<nav>``

BS3 commits: 8af8f7d5404536782f76e0ca02e01497f1e8b0ae
Expand advice about adding labels

BS3 commits: 353d8395aea20dad7d59bb54b8972a29ab2cb6e2
``<abbr>`` with title also useful for AT

BS3 commits: 202f1f8d69b119fd63d6788ee98e40321f53423b
fixes 15311: mention in docs requirement for .btn-group-* sizing on immediate .btn-group

BS3 commits: e11c634be998ccefef9587a059af680b17f9d9e2