Migrate documentation and base template to uv + pyproject.toml

[masenf]

Summary

All documentation examples and the base project template should use uv as the recommended Python package manager, replacing pip + requirements.txt with uv + pyproject.toml. This gives users strong dependency locking by default via uv.lock.

Motivation

• uv provides fast, reliable dependency resolution with a proper lock file (uv.lock) out of the box — no extra steps needed.
• requirements.txt does not support locking transitive dependencies without additional tooling (pip freeze, pip-tools, etc.), which most users skip.
• Aligning on uv improves supply chain security posture for the Python side of Reflex apps, complementing efforts to lock JS dependencies as well.
• uv is rapidly becoming the standard in the Python ecosystem and is what many users already expect.

Proposed Changes

1. Base template: Update reflex init templates to generate a pyproject.toml instead of (or in addition to) requirements.txt. Include reflex as a dependency with a version constraint.
2. Documentation: Update all installation and quickstart guides to use uv commands (e.g. uv init, uv add reflex, uv run reflex init) instead of pip install.
3. README / Getting Started: Reflect the uv-first approach in the repo README and any onboarding materials.
4. Backward compatibility: Continue to support pip and requirements.txt for users who prefer it, but make uv the recommended default.

[linear]

ENG-9270