ci: add Documentation workflow and build the existing docs sources

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

What changed and why

The repo already shipped docs/src/api.md but had no docs/make.jl, no docs environment, and no workflow, so the API reference was never built or rendered anywhere. Adds the missing docs environment (docs/Project.toml, docs/make.jl), a home page that renders the package docstring and a short usage section, and the standard reusable Documentation workflow including the token permissions the reusable requires (cf. SciML/DataCollocations.jl#54).

Each public name is rendered exactly once — the module docstring on the home page, the exported helpers on the API page — so Documenter's duplicate-docs check passes rather than reporting every export twice.

Verification (local, Julia 1.12.4)

After Pkg.develop(PackageSpec(path=pwd())) + Pkg.instantiate() exactly as CI performs:

[ Info: SetupBuildDirectory: setting up build directory.
[ Info: Doctest: running doctests.
[ Info: ExpandTemplates: expanding markdown templates.
[ Info: CrossReferences: building cross-references.
[ Info: CheckDocument: running document checks.
[ Info: Populate: populating indices.
[ Info: RenderDocument: rendering document.
[ Info: HTMLWriter: rendering HTML pages.
[ Info: Automatic `version="2.13.2"` for inventory from ../Project.toml

Zero errors. The page contains no @example blocks (it is a reference page for a test harness), so there is no unexecuted example; every documented name is rendered.

Not verified

• CI docs run on this PR (pending). Deployed rendering follows automatically once main docs complete.

🤖 Generated with OpenCode (model: muse-spark-1.3-contributor-free); no public session URL (local session at /home/crackauc/sandbox/tmp_20260917_040834_91074).