[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

searx/webutils.py

@@ -4,6 +4,7 @@ import csv
 import hashlib
 import hmac
 import re
+import inspect
 
 from io import StringIO
 from codecs import getincrementalencoder
@@ -125,3 +126,18 @@ def highlight_content(content, query):
                          content, flags=re.I | re.U)
 
     return content
+
+
+def is_flask_run_cmdline():
+    """Check if the application was started using "flask run" command line
+
+    Inspect the callstack.
+    See https://github.com/pallets/flask/blob/master/src/flask/__main__.py
+
+    Returns:
+        bool: True if the application was started using "flask run".
+    """
+    frames = inspect.stack()
+    if len(frames) < 2:
+        return False
+    return frames[-2].filename.endswith('flask/cli.py')