Update build tutorial

[zmitchell]

• Add a version to the Go build
• Update the example output from the build (may no longer see a signing line)
• Add description and version to the production build example
• Fix broken link to pulish-concept page
• In the publish section, remove the reference to needing a signing key/metadata-only upload
• Add the paid feature disclaimer to the "install" section since you need to have published the artifact first, which is a paid feature
• link to flox build and flox publish command reference pages

[ysndr]

Add a version to the Go build

do you mean pin go via flox install go@1.24.1?

[ysndr]

Update the example output from the build (may no longer see a signing line)

signing is still in there, i think that's macos signing from here: https://github.com/flox/flox/blob/4380522aa95bf1d811567b3dce83c8a63c6d57f8/package-builder/build-manifest.nix#L338C7-L338C27