docs: refresh README and add Sphinx/ReadTheDocs documentation portal

[falseywinchnet]

Motivation

• Make the repository top-level documentation follow a clear, library-style layout with simple quick-starts, feature list, and build/install guidance for C and C++ consumers.
• Provide a ReadTheDocs-ready Sphinx site so users and CI can build browsable API and user documentation from a single documentation/ folder.
• Capture and document the public C and C++ APIs (including BODFT) and make common maintenance tasks (local build, RTD setup) explicit.

Description

• Rewrote the top-level README.md into a concise library-style guide covering features, requirements, build (Make/CMake), install, package discovery, quick-start examples, concepts, examples/probes, and documentation pointers.
• Added a Sphinx documentation portal under documentation/ including index.md, README.md, installation.md, quick-start.md, concepts.md, examples.md, readthedocs-setup.md, contributing.md, and API reference pages api-c.md, api-cpp.md, and api-bodft.md documenting public types, plan lifecycle, transforms, layout conversion, and residue filters.
• Added Sphinx config and ReadTheDocs metadata: documentation/conf.py, documentation/requirements.txt, and documentation/.readthedocs.yaml (RTD config). The docs use myst-parser and the furo theme and are organized for easy sphinx-build and ReadTheDocs integration.
• Small documentation corrections: clarified the C++ RAII wrapper name (bfft::bodft) and made an explicit quick-start C example return-path fix so it is idiomatic and complete.

Testing

• Ran make test which built the library and tests and executed the test binaries; output: correctness ok backend=avx2-fma-256 and c api ok backend=avx2-fma-256 policy=fused-scatter-plus-layout-convert (tests passed).
• Ran git diff --check to ensure no whitespace/patch issues and it returned clean results.
• Attempted to validate docs build with python3 -m pip install -r documentation/requirements.txt && sphinx-build -W -b html documentation documentation/_build/html, but the environment failed to install Sphinx due to package index access returning 403 Forbidden, so sphinx-build could not be run in this environment.

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Codex Task