[mod] settings.yml: move wolframalpha to categ general & disable by default

As suggested in comment [1] as a good compromise between [2] and [3].

This will help users that want Wolfram Alpha instant answers by default with
every query and don't mind the slowness.  It will be disabled by default so it's
fast for all users, and those that need it can enable it as a user-configurable
option without requiring setting up their own instance to change the Wolfram
Alpha categorization.

[1] https://github.com/searxng/searxng/pull/813#issuecomment-1574851572
[2] https://github.com/searxng/searxng/pull/813#issuecomment-1020581765
[3] https://github.com/searxng/searxng/issues/966#issue-1171107451