[kammce]

Fixes the 📚 Deploy APIs workflow, which has failed on every push and release since the repo was created.

The shared api_docs_gen.yml workflow installs docs/requirements.txt and then runs conan hal docs, which runs Doxygen and then Sphinx in docs/. scatter_span had no requirements.txt and no Sphinx conf.py, so the job stopped at the install step.

Changes

• docs/conf.py, docs/requirements.txt, docs/_static/ and docs/_templates/: copied unchanged from strong_ptr. async_context has identical copies.
• docs/doxygen.conf: replaced the full default Doxyfile with the short config strong_ptr and async_context use. That config emits the XML Sphinx reads (the old one emitted HTML and LaTeX only) and names the project scatter_span instead of scatter span. conf.py builds the version switcher URL from that name: https://libhal.github.io/api/scatter_span/switcher.json.
• docs/index.rst and docs/api/: pages for scatter_span and its iterator, scatter_array and spanable, and for_each_run().
• .cspell.json: adds the doxygenclass, doxygenconcept and doxygenfunction directive names.

Testing

• conan hal docs --doc_version main ./docs builds locally with the pinned requirements (Sphinx 8.1.3).
• Every page renders its API.
• Doxygen still warns about members that aren't documented yet, as before. Those warnings don't fail the build.

[kammce]

Lgtm