Add checksum and signature info for ISO downloads

[j13k]

Would be helpful if the download instructions include SHA checksums and a digital signature - the current ISO is quite large, so providing this info would help with verifying download integrity and authenticity.

[EFrMG]

https://learn.omacom.io/2/the-omarchy-manual/93/security

The public key for all ISO signatures and Omarchy repo package is 40DFB630FF42BCFFB047046CF0134EE680CAC571 (verify at openpgp.org). The omarchy/omarchy-keyring package contains this as well and will be used to rollout any potential updates seamlessly.

You can find the signature for any ISO release by adding .sig to the URL. Like https://iso.omarchy.org/omarchy-x.x.x.iso.sig.

[j13k]

Thanks for confirming this.