Fix Context Menu samples a11y issues

[rolfsmeds]

• Adjust the color applied in the sample https://vaadin.com/docs/latest/components/context-menu#styling-menu-items to be WCAG compliant.
• Add a note about OS-specific context menu shortcuts (Shift-F10 on Win, Ctrl-Enter on Mac) not working in the embedded Context Menu samples due to technical reasons, but do work when sample opened in separate tab. (Or fix the issue if you know how)