Consolidate cloud image creation docs

[jimmykarily]

Right now we have various pages describing how one would create a custom cloud image. E.g.:

• https://kairos.io/docs/advanced/build/#build-a-cloud-image
• https://kairos.io/docs/reference/auroraboot/#generate-raw-disk-image-efi
• https://kairos.io/docs/advanced/customizing/

and there are not enough cross references or a full story that one can follow to create a cloud image from scratch. The process also misses the new kairos-init dockerfile completely (has a different ticket).

We need to consolidate the documentation so that there is a page one can visit and follow clear instructions on how to create a custom cloud image from scratch using the latest tools (kairos-init, auroraboot, etc).