PIPE-2738: Reorganize API sections and fix Task diagram
- Group domain/infrastructure and h*.tasks toctrees under a new
"Pipeline Modules" section with subsections.
- Rename "Full APIs" to "Complete API Reference".
- Raise docutils line_length_limit in conf.py setup() to fix the
Task Classes inheritance diagram being silently dropped.
PIPE-2738: Add sidebar hierarchy for automodapi and CLI docs
- Add cli_function_stubs Sphinx extension to generate per-function
stub pages and patch module stubs with hidden toctrees, giving
CLI functions proper sidebar nesting under their parent module.
- Move automodapi directives from inline apisummary.rst into
individual files under automodapi_src/ so each module becomes
a collapsible sidebar parent with its members nested beneath.
- Register cli_function_stubs extension and add _extension/ to
sys.path in conf.py.
- Simplify section titles in apisummary.rst, replacing inline
automodapi blocks with toctree references.
