Enhance downloader options and update documentation for OntoGraph

[ecarrenolozano]

This pull request introduces support for configurable downloader backends in the OntoGraph library, allowing users to select between different download mechanisms (such as "pooch" or "download_manager") either globally or per client. The documentation and tutorials have been updated to reflect these changes, and the codebase now passes downloader adapters through the main client classes for catalogs and ontologies.

Downloader backend configuration and usage:

• Added a DEFAULT_DOWNLOADER setting in ontograph/config/settings.py to specify the global default downloader backend, and exposed it in the public API. [1] [2]
• Introduced a get_default_downloader function in ontograph/downloader.py to instantiate the default downloader adapter based on settings or explicit override. [1] [2] [3]
• Updated both ClientCatalog and ClientOntology to accept an optional downloader parameter, which is passed through to all relevant operations, ensuring consistent downloader usage throughout the library. [1] [2] [3] [4] [5] [6]

Documentation and tutorial improvements:

• Expanded and clarified documentation in README.md, docs/index.md, and docs/learn/tutorials/quickstart.md to show how to configure and use different downloader backends, both globally and per client. [1] [2] [3] [4] [5] [6]
• Added a new tutorial, docs/learn/tutorials/tutorial0001_basics.md, demonstrating downloader configuration and usage scenarios, including catalog and URL downloads with both backends.
• Updated API reference docs for ClientCatalog, ClientOntology, and downloader modules to include downloader configuration and usage examples. [1] [2] [3]

Code quality and logging improvements:

• Improved logging throughout downloader adapters to provide more informative messages about backend selection, download progress, and error handling. [1] [2] [3]

These changes make OntoGraph more flexible and user-friendly when working in environments with different download requirements or preferences.