Add BlueBuild Silverblue recipe and install/rebase documentation

[Copilot]

This change introduces the baseline BlueBuild configuration needed to produce a Fedora Silverblue image from this repository, and documents how to build and install it on a Silverblue machine.

• BlueBuild recipe

  • Added a new recipes/recipe.yml as the repository’s canonical BlueBuild recipe.
  • Targets ghcr.io/ublue-os/silverblue-main and defines initial image metadata plus signing module scaffold.

• Silverblue install path documentation

  • Replaced placeholder README content with focused instructions for:
    • building the image via BlueBuild
    • rebasing an installed Silverblue host to the published image

• Reference commands

  bluebuild build ./recipes/recipe.yml
  
  sudo rpm-ostree rebase ostree-unverified-registry:ghcr.io/attraktorhh/computers:latest
  sudo systemctl reboot

Original prompt

I want to install a Silverblue machine using BlueBuild