Add check for "unknown" parameters in DocSearch config

I had a `min_indexed_lvl` parameter in a config, but the parameter is actually called `min_indexed_level`. But DocSearch scraper crawled the site as usual and I had no idea there is something wrong.

Is it possible to add a check which would raise a warning message when there are “unknown” parameters in the config?

Start by locating the DocSearch scraper's configuration parsing and parameter definitions. Check how configuration keys are currently handled, then add coverage for an unknown key such as `min_indexed_lvl`; done means the scraper warns about unrecognized parameters while continuing to identify the correctly named configuration.

Written by the indexing model from the issue text.