Updates to better align with ublue-os current state

[bsherman]

I did a test run with image-template and noticed a few things have fallen out of date.

1. we have dropped rpmfusion in favor of negativo17's fedora-multimedia repository, so our comments and examples need to reflect this.
2. we should emphasize how to customize aurora/bluefin, bazzite, ucore, but but right now the emphasis is on customizing main images... we shouldn't lose that, but make it clearer how to customize the above
3. we should consider using bind mount build pattern
4. we should consider using rechunker to enable re-adding of previously removed packages and space efficiency

[castrojo]

I'm working on #40 and have simplified this a bunch. Since bootc is just using dnf now we can just inline some examples in the containerfile:

• replacing a kernel (tulip says we need to get rid of cliwrap for this to work)
• adding a copr (done)
• removing packages from a base image (that image would need to be rechunked right?)
• Add a skeleton for some customization, perhaps an ADD usr usr and/or ADD etc etc

[xlionjuan]

If guide user how to use image-template with base image, it is kinda overlapping with BlueBuild.