[mod] webapp.py: update engines initialization condition

Always call initialize engines except on the first run of werkzeug with the reload feature.

the reload feature is activated when:
* searx_debug is True (SEARX_DEBUG environment variable or settings.yml)
* FLASK_APP=searx/webapp.py FLASK_ENV=development flask run (see https://flask.palletsprojects.com/en/1.1.x/cli/ )

Fix SEARX_DEBUG=0 make docs
docs/admin/engines.rst : engines are initialized
See https://github.com/searx/searx/issues/2204#issuecomment-701373438