docs: simplify README install and modernize image example

[Alek99]

Summary

• Collapse the README install/quickstart into a single uv-based block (mkdir / cd / uv init / uv add reflex / uv run reflex init / uv run reflex run).
• Move the All Thanks To Our Contributors section from README.md to the bottom of CONTRIBUTING.md.
• Remove the standalone License section from README.md (license info still lives in LICENSE).
• Update the image generation example to use gpt-image-1.5 and render the base64 data URL returned by the Images API.

Test plan

• git diff --check
• README renders correctly on GitHub
• mkdir my_app_name && cd my_app_name && uv init && uv add reflex && uv run reflex init && uv run reflex run works on a clean dir
• CONTRIBUTING.md renders the contributors graphic at the bottom
• Updated example code runs and renders the generated image via data URL

Generated with Claude Code

[codspeed-hq]

Merging this PR will not alter performance

✅ 9 untouched benchmarks

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_{Comparing docs/readme-cleanup (163f027) with main (7a7a4d3)}