feat: create a backend to parse USPTO patents into DoclingDocument

[ceberam]

Resolves #605

This PR implements the following changes:

• Add a backend implementation to parse patent applications and grants from the United States Patent Office (USPTO).
• Refactor the module docling/datamodel/document.py to address the scenario of multiple InputFormat instances with the same mime type. In particular, add a function that further examines part of an input document to guess the InputFormat instance to use for the conversion.

This PR is intended to be merged after #557

Checklist:

• [x] Documentation has been updated, if necessary.
• [x] Examples have been added, if necessary.
• [x] Tests have been added, if necessary.

Limitations:

The following points will need to be addressed in later PRs:

• Slightly refactor guess_format function in docling/datamodel/document.py module, once we support another XML InputFormat, since application/xml mime type will already be ambiguous.
• Add an abstract static method in abstract_bakend.py that examines a partial content of a document and determines if the backend implementation supports a document type with that content. This function could then be called in docling/datamodel/document.py module and avoid duplicated code when disambiguating mime types.
• Add documentation and notebook examples.
• Eventually create a default text/plain backend parser.

[mergify[bot]]

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