Added BacktraceClient.addAttribute method

[rick-bt]

Description

• Added text to describe the new BacktraceClient.addAttribute method
• Adjusted Global Custom Attribute text to align with the new section
• Changed kotlin snippet formatting to java so it can be pretty, too

Types of Changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation fix (typos, incorrect content, missing content, etc.)

[github-actions[bot]]

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