require/include php file in add_menu()

You can turn on output buffering, include (and evaluate) the PHP file, and save the output (of the evaluated code) in a variable, like so:

ob_start();
include 'docs/row_layouts.php';
$html = ob_get_clean();

Then just use 'html' => $html in the meta array when you call the $admin_bar->add_menu().

Or if you don’t need to evaluate any PHP code in the file, you could use file_get_contents():

'html' => file_get_contents( 'docs/row_layouts.php' )

And you may need to or better use a full absolute path.