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

shorter, neater, well supported
Make sidebar navigation actual ``<nav>``
Expand advice about adding labels

Harmonizing this with same advice given for CSS inline forms
``<abbr>`` with title also useful for AT

...and remove the redundant paragraph for basic abbreviation, which
simply repeats what was just said above.
Merge pull request #15320 from patrickhlauke/docs-accessibility-tweaks

Docs accessibility tweaks: use ``aria-label`` for close ``<button>``, expand advice for ``<abbr>``, form control labels, make sidebar navigation a ``<nav>``