New page on ML environments

[aperloff]

Write some documentation which serves as a centralized location for listing ML environments/images. Some ideas would be:

• Point to LCG documentation and list some images or types of images which have ML software. In this case, will also need to mention the types of systems which can use these LCG images.
• Introduce unpacked.cern.ch
• Point to the LPC Docker images and Singularity images
• Other ideas?
• Might want to spend some time talking about the basic setup of Python/Conda environments or Docker/Singularity containers.

@chernyavskaya @thaarres

[aperloff]

• FNAL LPC GPU Software Environments: code, Docker Hub, sync
• LCG/SWAN: list of releases

[aperloff]

• Coffea Dask image w/ TensorFlow + PyTroch: GitHub, Docker Hub