Singularity container support for run script

[fpjentzsch]

Extends run-docker.sh to support execution of FINN inside a Singularity environment instead of Docker. Expects the Singularity image to be pre-built (from the normal Docker image) by the user. We use a GHA in a different repo for this.

I also added some documentation to the "Getting Started" page and a GHA that tests this feature by building a Singularity container from the FINN Docker image and running quicktest inside it.

Example usage:

1. Set the new env var to the Singularity image (path or registry URL) you want to use:
   export FINN_SINGULARITY=oras://ghcr.io/eki-project/experiment-manager/finn_apptainer_xilinx:dev
   (this points to a public nightly build of the official dev branch)

2. Run run-docker.sh normally

[auphelia]

Thanks @fpjentzsch !