feat: enhance attachment functionality in Qase Behave

[gibiw]

• Updated the attachment methods to allow for step-specific attachments using qase.attach_to_step().
• Improved the QaseFormatter to manage current step attachments more effectively.
• Added documentation for attaching files and content to both test cases and specific steps, enhancing usability and clarity for users.

These changes improve the flexibility and functionality of the attachment system in Qase Behave, allowing for better tracking of test step-related data.

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Note

Introduces step-level attachments and integrates them into result reporting.

• Adds qase.attach_to_step() with a pending-attachment queue applied when a step becomes active; refactors attachment creation into _create_attachment() and maintains scenario-level qase.attach()
• Updates formatter to set/clear current_step per step, applying pending attachments and ensuring correct association; clears step on new scenario
• Updates docs (docs/usage.md) with test- and step-level attachment examples and parameter details; updates example step to use attach_to_step; bumps version to 2.0.6 and adds changelog entry

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