docs: improve documentation and fix examples

[denisvmedia]

Description

This PR improves the documentation and fixes examples to ensure consistency and accuracy across the codebase.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Code refactoring
• Test improvement

Related Issues

N/A

Changes Made

• Updated README.md attributes example to use correct errx.Classify() API instead of deprecated attrs.Tag()
• Added documentation for new API methods: FromAttrMap, DisplayTextDefault, ToSlogAttrs, ToSlogArgs
• Enhanced package documentation in errx.go with subpackages overview section
• Fixed example comment to consistently use 'sentinel' terminology instead of 'tag'
• Removed outdated Format() reference from CHANGELOG.md

Testing

Test Coverage

• All new code is covered by tests (N/A - documentation only)
• All existing tests pass (go test ./...)
• Tests pass with race detector (go test -race ./...)

Manual Testing

Documentation changes verified by reviewing rendered markdown and ensuring code examples are accurate.

Documentation

• Code is documented with godoc comments
• README updated (if applicable)
• CHANGELOG updated
• Examples added/updated (if applicable)

Code Quality

• Code follows the project's style guidelines
• golangci-lint passes (golangci-lint run)
• go vet passes
• go fmt has been run
• No new warnings introduced

Breaking Changes

Does this PR introduce breaking changes?

• Yes
• No

Performance Impact

• No performance impact
• Performance improvement (include benchmarks)
• Potential performance regression (explain why acceptable)

Additional Notes

These documentation improvements ensure that users have accurate examples and complete API reference information.

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the code style of this project
• I have added tests that prove my fix/feature works
• All tests pass locally
• I have updated the documentation accordingly
• I have added an entry to CHANGELOG.md