added API docs and demo

[alexlib]

This pull request introduces new documentation and supporting assets to explain and demonstrate the unified Python API approach in openptv-python, emphasizing how the library transparently routes operations to pure Python, Numba, or native optv backends. It adds a detailed explainer page, a script to generate machine-specific benchmark artifacts, and example benchmark results to the documentation.

The most important changes are:

Documentation Enhancements:

• Added a comprehensive documentation page (native_backend_unification.md) describing the unified backend model, current dispatch status, and rationale for a single public Python API that transparently selects the best available backend. This page includes pipeline diagrams, a summary table of benchmark results, and guidance for users and contributors.
• Updated the documentation index in index.md to include the new backend unification page.

Benchmark Demo and Artifacts:

• Added a new script (generate_native_stress_demo.py) that runs the native stress benchmark suite, parses the output, and generates machine-specific artifacts: a JSON summary, an SVG chart, and a raw log. This script automates the process of updating benchmark results in the documentation.
• Added an example JSON benchmark result file (native-stress-demo.json) to the documentation's static assets, showing detailed timings and speedups for each workload stage.

[Copilot]

Pull request overview

Adds documentation and helper scripts to explain and demonstrate the unified Python API/backends approach, including a machine-specific native stress benchmark demo rendered into the docs.

Changes:

• Added a new docs page describing backend unification and current dispatch status, including an include of generated benchmark results.
• Added scripts to (a) run/parse the native stress benchmark into JSON/log and (b) render that JSON into a Markdown include used by the docs.
• Updated docs index, root Makefile docs build, contributor workflow notes, and gitignore to support the new docs assets.

Reviewed changes

Copilot reviewed 7 out of 9 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
docs/render_native_stress_demo_include.py	Renders a Markdown include from the committed benchmark JSON.
docs/generate_native_stress_demo.py	Runs pytest stress benchmark and emits machine-specific JSON/log artifacts.
docs/native_backend_unification.md	New explainer page describing unified backend dispatch + embeds generated demo include.
docs/index.md	Adds the new explainer page to the docs index.
docs/_static/native-stress-demo.json	Adds an example benchmark output JSON for the docs demo.
README.md	Updates contributor workflow guidance (uv-first + conda alternative).
Makefile	Adds uv env update targets; updates docs-build to generate the md include.
.gitignore	Ignores docs/_generated/ outputs.

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