Fetch Python intersphinx data from $SPHINX_PYTHON_OBJECTS

Builds requiring net access are clearly *very* bad.
Sync cupage.cmdline API doc
Fix inter-document references
Add appendix for objects unavailable via intersphinx
[QA] Typo fix in usage.rst
Merge branch 'feat/doc_improvements'

* feat/doc_improvements:
  [QA] Typo fix in usage.rst
  Add appendix for objects unavailable via intersphinx
  Fix inter-document references
  Sync cupage.cmdline API doc
  Fetch Python intersphinx data from $SPHINX_PYTHON_OBJECTS