update docs for new OS and usage of `denv`

[tomeichlersmith]

kinda self-explanatory, I haven't re-written "how to use these images" in a while and I think it will be helpful to go through the entire manual once before making another major release

[tomeichlersmith]

I'd like to add a helper for pull by digest. The CI posts the digests of any successful builds and so if the build that matches your architecture is done and you want to jump into testing without waiting for the other one, you can do this.

I'm using podman.

1. Download the artifact for the appropriate architecture.
2. Unpack archive to get empty file with digest name, use digest to pull built image and then tag it with something helpful.

cd ~/Downloads
mkdir digest
cd digest
unzip ../amd64-digest.zip # or arm64-digest.zip if downloaded arm
podman pull ldmx/dev@sha256:$(ls *)
podman tag ldmx/dev@sha256:$(ls *) ldmx/dev:some-helpful-name
cd ..
rm -r digest amd64-digest.zip