Complete and clarify development setup documentation

[wdconinc]

The current developer setup process for mollerdb is fragmented across several files (README.md, docs/README.md, schema/README.md) and lacks a unified, complete, and easy-to-follow guide. This makes onboarding new contributors and ensuring reproducible development environments difficult.

Summary of missing pieces:

• No single, comprehensive guide for setting up a C++ and Python dev environment (steps are scattered and sometimes incomplete).
• Native dependencies (libpqxx, Arrow, pybind11, CMake, compiler versions) are not fully or clearly documented for all platforms.
• No single step-by-step workflow for cloning, submodule setup, schema generation, building/testing C++ and Python, or validating the environment.
• Troubleshooting advice for common build errors is missing.
• No checklist for verifying the setup (running tests, building wheels, schema integration).
• No reference to Docker-based setup (see issue Add development Dockerfile with all dependencies for mollerdb #27).

Deliverables (Definition of Done):

• Add a single, discoverable dev setup guide (e.g., docs/DEV_SETUP.md or a section in docs/README.md) that includes:
  • System and Python prerequisites for Linux/macOS (and Windows, if supported)
  • Step-by-step setup: clone, submodules, dependencies, build, test for both C++ and Python
  • Troubleshooting section for common issues
  • Reference to Docker-based setup (link to issue Add development Dockerfile with all dependencies for mollerdb #27)
  • Checklist of commands to verify a working setup
• Update README.md and docs navigation to point to the new guide
• Confirm that a new contributor can follow the guide from scratch to a working environment

References:

• README.md
• docs/README.md
• schema/README.md
• Phase 0 tracking issue #19
• Dockerfile issue #27