docs: add Sourcey C++ API reference

[zdfgu113]

Summary

• generate a source-linked C++ API reference from Kiwi's public headers with Doxygen and Sourcey 3.6.5
• publish the generated reference under the existing Read the Docs site at /cpp-api/
• link the reference from the main documentation navigation

Verification

• doxygen Doxyfile.sourcey
• sourcey build --output sourcey-preview-113b (18 pages generated)
• python -m sphinx -W -b html docs/source build/sphinx-113
• durable preview: https://zdfgu113.github.io/kiwi/

The Sourcey config pins source links to commit e8acf1e0eb8c21c6193d7dd621e3f03b7277f122. Generated HTML is not committed; Read the Docs builds it after the Sphinx build.

Closes #60.

AI assistance disclosure: OpenAI Codex assisted with generation, configuration, and testing; the resulting files and public output were reviewed and verified locally.

[MatthieuDartiailh]

It is unfortunate the docs are built as separate pages. It would be much nicer to embed the doxigen generated docs into the sphinx docs. A quick search suggest breathe and doxisphinx could be two possible paths.

[zdfgu113]

Thanks for the suggestion. I updated the PR to use Breathe so the Doxygen XML is rendered directly inside the existing Sphinx C++ API page instead of appearing as a separate top-level docs section. Doxygen now runs in Read the Docs' pre-build phase, and the same XML still feeds the searchable Sourcey reference. I verified the change locally with python -m sphinx -W -b html docs/source build/sphinx-113-breathe; the build completes without warnings.

[MatthieuDartiailh]

Looks better. Could you remove the hardcoded URL to the C++ API docs and met sphinx build that link ?