[DOCS] Final report and usage documentation

[hubertwojcik]

Goal

Write the final project documentation covering algorithm design, implementation decisions, performance results, and how to reproduce everything from scratch.

Tasks

• Write docs/report.md: algorithm overview, parallelization strategy per implementation, boundary handling approach
• Add performance results section: include all four plots and a summary table (peak GB/s per implementation)
• Write docs/usage.md: how to build, generate data, run each implementation, and reproduce benchmarks
• Update root README.md with quick-start, prerequisites, and links to docs

Acceptance Criteria

• docs/report.md covers all four implementations with results
• A new user can follow docs/usage.md end-to-end without additional guidance
• README includes build badge, quick-start, and links to report