Improve repository documentation

[nsergej]

Summary

• Rewrote README.md with clearer usage, IPv4-only limitations, repository layout, sample outputs, troubleshooting, and detailed Delphi build instructions.
• Added CONTRIBUTING.md, SECURITY.md, and CHANGELOG.md.
• Renamed docs/sample_repor.* to docs/sample_report.* and synchronized README examples with the docs sample files.

Validation

• Checked that local Markdown links resolve.
• Checked external README/CHANGELOG URLs returned HTTP 200.
• Verified README sample values match docs/sample_report.csv and docs/sample_report.json.
• Parsed docs/sample_report.csv and docs/sample_report.json successfully.
• Parsed docs/sample_report.html as HTML and checked expected sample values.
• Ran git diff --cached --check.
• Confirmed no staged changes touch src/PortScanner.dpr, src/Unit1.pas, or src/Unit1.dfm.
• Built Win32 with cgrc + dcc32 in a temporary copy.
• Built Win64 with cgrc + dcc64 in a temporary copy.

Note: Delphi reports the existing W1050 warning in Unit1.pas during both builds; no source code was changed in this PR.