Add (geo) data cube specification and cube generation guide to docs

[forman]

Is your feature request related to a problem? Please describe.

xcube has a quite clear idea of how a (geo) data cube should be represented using Zarr as external format and xarray.Dataset as in-memory representation.

Describe the solution you'd like

Provide a specification of the xcube (geo) data cube profile and provide a practical guide to generate cubes that conform to the specification. Add them as markdown files to xcube's docs folder.

The practical guide should include how-to's such as

• how to encode datatype and set fill value (before using xarray and afterwards using "xcube patch")
• how to encode grid mapping (before using xcube/xarray and afterwards using "xcube patch")
• how to encode color mappings (before using xcube/xarray and afterwards using "xcube patch")
• how to determine a variable's reasonable display ranges taking into account data distribution (linear, normal, log) and categorical data
• how to fix issues with time dimension/coordinates
• how to create pyramids
• how to write cubes without fill value chunks

Additional context

• Use the dataset specifications we provided for the ESA DeepESDL and AVL projects.
• OGC Seeks Public Comment on Creating new GeoDataCube Standards Working Group