fix(a11y): give embedded videos an iframe title (#2141)

Screen readers announce an iframe by its title. The video shortcode
emits title="" when the author omits the title argument, so these two
embeds failed axe's frame-title check (serious, WCAG 2.0 A, SC 4.1.2).

Add title="CERN: The Journey of Discovery" to the two live {{< video >}}
calls, and to the escaped copies the same pages display as source, so
the shown example matches the rendered output. In tables.qmd the call
sits inside a data-qmd="..." attribute, so the title uses single quotes.

Author: cwickham