Adopt src package layout and add dev packaging helpers

[srichs]

Motivation

• Standardize the project to the common src-layout so installed packages don't accidentally import from the repository root during development.
• Provide minimal packaging compatibility for tools that expect a setup.py and a convenient way to install development extras.
• Ensure docs build and tests import the package correctly from the new layout.

Description

• Moved the spot_api package into src/spot_api to adopt the src layout and left public exports unchanged.
• Updated pyproject.toml to add [tool.setuptools] package-dir and [tool.setuptools.packages.find] where = ["src"], and added [tool.pytest.ini_options] pythonpath = ["src"] for local test runs.
• Added a minimal setup.py shim with setup() for compatibility with tools that rely on a setup.py entrypoint.
• Added requirements-dev.txt containing -e .[docs,test,lint] and updated README.md to document using it, and modified docs/conf.py to insert src on sys.path for Sphinx imports.

Testing

• Ran pytest which passed all tests (119 passed).
• Attempted python -m pip install -e . but it failed in this environment because build dependency downloads were blocked by network/proxy restrictions (could not fetch setuptools>=61.0).
• Verified the package imports locally for tests and documentation generation after updating sys.path in docs/conf.py.

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Codex Task