swh-docs: Add storage sites documentation (v3)

Indicate where Softwareheritage stores its data, organize by
big storage locations:

• Rocquencourt physical machines

• Rocquencourt virtual machines

• Azure-euwest

• Amazon web services

• Object storage is important and gets its own dedicated document

• So do the hypervisors

Also link to the existing Elasticsearch document

Reviewed-by: ardumont, vlorentz