Docs/mini geozarr spec

[d-v-b]

This PR adds a compact form of the GeoZarr spec that will define exactly the interpretation of GeoZarr that will be implemented by this library. This mini spec is compatible with the current editor's draft GeoZarr spec, but is much smaller and uses language that is more directed for guiding an implementation.

I am opening this in draft form so @emmanuelmathot can suggest additional constraints / rules / metadata that I have not included yet. For example, there are some minimal CF metadata fields that should probably be specified for downsampled Datasets (like "grid_mapping").

This spec diverges somewhat in recommending that collections of downsampled datasets (which I am calling "Multiscale Datasets") do not contain anything other than the downsampled datasets. This will make parsing easier, but it may also complicate some applications, so it would be great to know if this blocks off intended usage.

To illustrate what this means:

This layout would be permitted:

/satellite_imagery/
├── 7/                       # Zoom level 7 (TileMatrix id "7")
│   ├── red_band
│   ├── green_band
│   └── blue_band
├── 8/                       # Zoom level 8 (TileMatrix id "8") 
│   ├── red_band
│   ├── green_band
│   └── blue_band
├── 9/                       # Zoom level 9 (TileMatrix id "9")
│   ├── red_band
│   ├── green_band
│   └── blue_band
...
├── 14/                      # Zoom level 14 (TileMatrix id "14")
│   ├── red_band
│   ├── green_band
│   └── blue_band
└── 15/                      # Zoom level 15 (TileMatrix id "15")
    ├── red_band
    ├── green_band
    └── blue_band

But this layout would be discouraged, and the code in this repo would not parse / generate it:

/satellite_imagery/
├── red_band # Zoom level 0, not inside a separate Dataset
├── green_band
├── blue_band
├── 7/                       # Zoom level 7 (TileMatrix id "7")
│   ├── red_band
│   ├── green_band
│   └── blue_band
├── 8/                       # Zoom level 8 (TileMatrix id "8") 
│   ├── red_band
│   ├── green_band
│   └── blue_band
├── 9/                       # Zoom level 9 (TileMatrix id "9")
│   ├── red_band
│   ├── green_band
│   └── blue_band
...
├── 14/                      # Zoom level 14 (TileMatrix id "14")
│   ├── red_band
│   ├── green_band
│   └── blue_band
└── 15/                      # Zoom level 15 (TileMatrix id "15")
    ├── red_band
    ├── green_band
    └── blue_band