docstrings

@clima

Add or normalise Julia docstrings on public symbols (exported types, functions, and constants) so the package's public API is fully self-documenting and the Documenter.jl docs build passes its checkdocs check. After writing docstrings, also updates docs/src/API/ pages so every exported symbol appears exactly once, organised into logical categories, with stale entries removed. Invoke this skill whenever the user mentions: docstring, missing doc, undocumented symbol, API doc, checkdocs warning, docs/src/API, @docs block, or asks to document a type or function, sync the API pages, or keep the API index up to date. Also use it when the user asks to "write docs for" or "add docs to" source files, or when a CI failure mentions missing or incomplete docstrings.