release ISO images with GitHub release artifacts

[mmguero]

In some places in the documentation we say something like:

While official downloads of the Malcolm installer ISO are not provided ...

and

Official downloads of the Hedgehog Linux installer ISO are not provided: however, it can be built easily on an internet-connected Linux host with Vagrant

etc.

The reason we don't post the releases in GitHub is because of release file size limitations.

I think we need to just start releasing the images using split to split them into < 2GB chunks.

So here's what I think we need to build:

1. both a bash and PowerShell script that takes the release ISO and splits it into < 2GB chunks ready for release (as well as generating a text file containing the sha256 sum to be included in the release as well)
2. both a bash and PowerShell script that takes the split ISO pieces and reassembles them and double-checks the resultant reassembled file against the sha256 sum txt file

These scripts need to be as portable as possible (e.g., the bash script should work across Linuxes and macOS and probably even WSL2 and the powershell script should work on windows 10 and up without external dependencies).

[mmguero]

Done, releases from v23.03.1 forward will have ISOs on the GitHub download page.