Add tabindex=0 to a table

[rianrietveld]

From the accessibility audit, issue #232

SC 2.2.2 keyboard: Tables can overflow horizontally, the container needs to be focusable so keyboard-only users can use the arrow keys to scroll sideways. Similar to the code blocks issue. We saw table overflow occur in Safari when we zoomed the page https://wpaccessibility.org/docs/topics/core/ to 400%.

The work:
Add tabindex="0" to <div class="table-wrapper">, this can probably be done in wp-a11y-docs.js.