Following the pattern from ROCm/rocm-libraries#2679, add logic to skip
CI builds when only documentation files are modified.
Changes:
- Add SKIPPABLE_PATH_PATTERNS for docs, markdown, and .gitignore files
- Return empty projects list when only skippable paths are modified
- No workflow changes needed - existing projects != '[]' check handles it
- Add unit tests for doc-filtering logic
- Fix existing tests with proper subprocess mocking
Reference: https://github.com/ROCm/rocm-libraries/pull/2679