reword "{Download,Extract} rootfs tarball" sections

[sighook]

The following sections of the documentation contain a hardcoded version of rootfs:

• https://zeppe-lin.github.io/handbook.7.html#Download-rootfs-tarball
• https://zeppe-lin.github.io/handbook.7.html#Extract-rootfs-tarball

It would be better to make the text more generalized with a link to the page with the latest release. Then there is no need to change the version in the handbook with each new release of rootfs