website: fix sub-menu in the header for mobile devices

If the browser width is less than 1300 pixels: click top left
on "About" and the sub-menu will not open anymore due the missing IDs.

PR: 297509
Approved by: carlavilla, ziaee
Fixes: 10633eb86b6396ec3fe8b666f8e250d67398fbec
Differential Revision: https://reviews.freebsd.org/D60236