DOC: Surface curated module-level descriptions on the tools API pages

Background

Follow-up from #850. That PR repointed the base-level tools API pages to document the public objects in the quantecon namespace (e.g. quantecon.ARMA) using .. currentmodule:: quantecon plus explicit autoclass / autofunction directives, instead of .. automodule::. This fixed a regression where the pages had started rendering private headings like class quantecon._arma.ARMA(...).

A deliberate side effect: because these pages no longer use automodule, the module-level docstring is no longer rendered on them. Previously automodule would show the module's summary (e.g. "Provides functions for working with and visualizing scalar ARMA processes") at the top of the page. The per-object docstrings still render fully — only the short module-level blurb is gone.

This issue tracks optionally reintroducing a short, user-facing module description at the top of each shim-backed tools page.

Proposed approach

Have docs/qe_apidoc.py read the implementation module's docstring via ast.get_docstring(...) and emit only its leading summary paragraph (up to the first blank line) as prose beneath the page heading, before the currentmodule directive. Taking just the summary paragraph keeps the output user-facing and naturally excludes the multi-line developer TODO: sections that trail a couple of these docstrings. This keeps the pages generated and low-maintenance rather than hand-curated.

Audit of the 21 shim-backed modules

Status of each quantecon/_<name>.py module docstring today:

Suggested tasks

• Add a leading summary paragraph to each shim-backed tools page, generated from the implementation module's docstring.
• Add short module docstrings to _lqnash.py and _rank_nullspace.py (currently none).
• Optionally improve the terse _filter.py summary.
• Regenerate the committed stubs and confirm a warning-clean Sphinx build.

Low priority / nice-to-have — the current pages are already correct and complete at the object level.

Start with docs/qe_apidoc.py and inspect how the 21 shim-backed modules are mapped to the tools API pages. Check _lqnash.py and _rank_nullspace.py for missing module docstrings, then regenerate the committed stubs and run a warning-clean Sphinx build; done means the pages show only each module's leading summary while public object documentation remains intact.