Columnar faceting: Python user guide and example #293

Part of #289 (columnar faceting in the Python bindings).

Scope

Add a guide page to the Python docs: creating a COLUMNAR index, the FACETS clause with every facet kind, reading facets from a result set, the unavailable reasons, facet.searchValues, GROUP BY acceleration and its setting, and the known limitations (negative zero in MIN/MAX, DISTINCT above PRECISION n is approximate). Link it from the main docs and the README feature list, with one example in examples/.

Depends on the other sub-issues of #289.