Convert Add group link to button

Convert Add group link to button

The "Add group" peforms an on-page action and does not route or link to
anywhere else hence, not semantically a link but a button.

This commit implements the ehancement described at the respository level.

A change would be required in [@nextcloud/nextcloud-vue](https://github.com/nextcloud/nextcloud-vue/issues/4108)
since the `NcAppNavigationNewItem` is an out-of-repo dependency.

Signed-off-by: fenn-cs <fenn25.fn@gmail.com>