Update arm architecture

The aarch64 architecture is one of the primary arches. We should update the docs to reflect that. We should also mention the difference between aarch64, armhf and armv7

https://gitlab.alpinelinux.org/alpine/docs/user-handbook/-/blob/b0b43a026ea3e6a920568c379ae9ed3d696fd553/modules/Installing/pages/medium.adoc#user-content-architecture