Support dataset- and variable-descriptions as Markdown files

[TejasMorbagal]

Currently, dataset and variable descriptions in xcube viewer are typically stored as plain strings in dataset metadata or in-line markdown in server config. While this works for short notes, it becomes limiting for more detailed documentation.

It would be very useful if xcube could support descriptions provided in Markdown format. This would keep the server config cleaner and precise.

DataStores:
  - Identifier: xcube-examples
    StoreId: s3
    StoreParams:
      root: xcube-examples
      storage_options:
        anon: true
    Datasets:
      - Path: demo.levels
        Description: dataset-desc/demo.md