DOC: Rebuild files every time in `make html`
DOC: Fixes from #38ab61269
ENH: Math/BLAS/LAPACK Cython declarations portable
DOC: File docstring typo: ARMA -> SARIMAX
DOC: Git ignore Sphinx autosummary generated files
FIX: Fixes model summary observations text.
FIX: `start_params` when trends are present.
FIX: `predict` when trend is present.
ENH: Add Kalman gain; cache calculated variables
FIX: notebook images now have unique filenames
REF: Refactored Cython code into separate modules.