Rework README

Previews

• GitHub https://github.com/postgres-ai/database-lab-engine/blob/rework-readme/README.md
• GitLab https://gitlab.com/postgres-ai/database-lab/-/blob/rework-readme/README.md

Changes

• Added CODE_OF_CONDUCT.md
• Added CONTRIBUTING.md
• Added SECURITY.md
• Changed logo graphics (wide logo, new font, Cuprum)
• New section: Features
• Animated GIF demonstrating fast cloning of 1 TiB database
• Balance between GitHub and GitLab (the latter is supposed to be source of truth for the code, but both are fine for communicating with community)
• "Powered by" graphics for dark and light backgrounds
• Brief license description
• More words about Community throughout the texts

TODO

• MR desc
• proofread
• polish graphics
• badges