ci: add Documentation workflow and docs sources

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

What changed and why

This repo had no documentation CI and no docs sources at all, so none of the premade prob_* problems were documented anywhere. Adds:

• docs/ with a home page (what the package is, the seven problem sublibraries, installation, and one runnable example) and an API reference that renders the docstrings of all seven sublibraries.
• The standard reusable Documentation workflow, including the token permissions the reusable requires (cf. docs: execute the DataInterpolations tutorial examples in the build DataCollocations.jl#54).

docs/Project.toml pins the seven sublibraries (all registered), the umbrella package and Documenter; the umbrella package itself is Pkg.developed by path by CI as usual. docs/build/ is now gitignored.

Verification (local, Julia 1.12.4)

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

[ Info: Populate: populating indices.
[ Info: RenderDocument: rendering document.
[ Info: HTMLWriter: rendering HTML pages.
│     HTML file:           api.html
[ Info: Automatic `version="5.3.0"` for inventory from ../Project.toml

grep -cE "failed to run|ERROR" → 0. The home page has one executing @example problem-library block (solve(prob_ode_linear, Tsit5()) printing its retcode); it executed as part of this build, so no example is left unexecuted.

Not verified

• CI docs run on this PR (pending). Deployed rendering follows automatically once master docs complete.
• The larger API page renders every docstring in the seven sublibraries; I verified it renders and the build completes, but did not eyeball each rendered entry.

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