Update docs to reflect new API changes

[aliddell]

Includes:

• Set output key and overwrite control acquire-zarr#106
• Expose method used to downsample arrays acquire-zarr#108
• fix: is_connection_valid in s3.connection.cpp acquire-zarr#109 (support for IAM and config file options for S3 authentication)
• Deprecate Zarr V2 acquire-zarr#110
• Add serializable downsampling methods with reproducible metadata acquire-zarr#118
• Multi-array streaming acquire-zarr#128
• Add a close() method to ZarrStream acquire-zarr#130