document public container classes in the api reference - #1192

fixes #1122.

adds an api reference page for construct.Container and construct.ListContainer, linked from the main api table of contents. documenting the public classes explicitly gives downstream intersphinx users entries for both types instead of relying on the construct.lib automodule page, which omits them.

validation: built the complete html documentation with sphinx 8.2.3 and confirmed both class entries and target anchors in objects.inv and the rendered page. a separate sphinx project successfully resolves both cross-references with -n -W using that inventory. the complete documentation has the same 54 pre-existing warning messages before and after the change; no new warnings. git diff --check passes.