Add E4S container and build cache information to ExaGO documentation

[cameronrutherford]

Per @eugeneswalker, E4S has useful build caches and container base images that users can leverage, and so we should document that appropriately:

I've set up a public repo for running nightly Spack development builds of exago@develop for the following GPUs on UO Frank systems:

• https://gitlab.e4s.io/uo-public/exago

The last workflow run can be seen from the pipelines page:

• https://gitlab.e4s.io/uo-public/exago/-/pipelines

• https://gitlab.e4s.io/uo-public/exago/-/pipelines/9711

• https://gitlab.e4s.io/uo-public/exago/-/jobs/223788(AMD MI210 Job)

• https://gitlab.e4s.io/uo-public/exago/-/jobs/223787 (NVIDIA A100 Job)

The workflow linked above uses the following CI container images:

• esw123/exago-rocm90a:2023.10.23 (for AMD MI210)
• esw123/exago-cuda80:2023.10.23 (for NVIDIA A100)

These container images are built using recipes from the following public repository:
https://gitlab.e4s.io/uo-public/exago-images

CUDA Image:

• https://gitlab.e4s.io/uo-public/exago-images/-/tree/main/images/cuda
• https://gitlab.e4s.io/uo-public/exago-images/-/blob/main/images/cuda/Dockerfile
• https://gitlab.e4s.io/uo-public/exago-images/-/blob/main/images/cuda/spack.yaml

ROCM Image:

• https://gitlab.e4s.io/uo-public/exago-images/-/tree/main/images/rocm
• https://gitlab.e4s.io/uo-public/exago-images/-/blob/main/images/rocm/Dockerfile
• https://gitlab.e4s.io/uo-public/exago-images/-/blob/main/images/rocm/spack.yaml

The binaries used to generate the images above are publicly exposed via standard Spack mirror here:

$ spack mirror add exago-ci https://cache.e4s.io/exago-ci
$ spack buildcache keys -it
$ spack buildcache list -al

The idea is that the CI images contain all the dependencies needed to build ExaGO from source, already installed.

This is what I've been working on. Right now it just runs the basic ctest once the spack dev-build completes.

[cameronrutherford]

https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry#authenticating-with-a-personal-access-token-classic - we also have ghcr build cache from #85, so these should both be documented