fix schema

[fmigneault-crim]

Added

• Added example model architecture summary text.

Changed

• Modified $id if the extension schema to refer to the expected location when eventually released
  (https://schemas.stacspec.org/v1.0.0-beta.3/extensions/dl-model/json-schema/schema.json).
• Replaced dtype field by data_type to better align with the corresponding field of
  raster:bands.
• Replaced nodata_value field by nodata to better align with the corresponding field of
  raster:bands.
• Refactored schema to use distinct definitions and references instead of embedding all objects
  within dl-model properties.
• Allow schema to contain other dlm:-prefixed elements using patternProperties and explicitly
  deny other additionalProperties.
• Allow class_name_mapping to be directly provided as a mapping of index-based properties and class-name values.

Deprecated

• Specifying class_name_mapping by array is deprecated.
  Direct mapping as an object of index to class name should be used.
  For backward compatibility, mapping as array and using nested objects with index and class_name properties
  is still permitted, although overly verbose compared to the direct mapping.

Removed

• Field nodata_value.
• Field dtype.

Fixed

• Fixed references to other STAC extensions to use the official schema links on https://stac-extensions.github.io/.
• Fixed examples to refer to local files.
• Fixed formatting of tables and descriptions in README.