[doc] recommend to use 'use_default_settings=True'

Since #2291 is merged, it is recommend to use::

  use_default_settings=True

1. Add a template file use_default_settings.yml::

    SEARX_SETTINGS_TEMPLATE="${REPO_ROOT}/utils/templates/etc/searx/use_default_settings.yml"

2. In Chapter "Configuration" recommend to make use of
   'use_default_settings=True' and describe it

3. Rewrite of docs/admin/settings.rst
   - move chapter 'settings.yml location' to the top
   - update and split chapter 'Global Settings'

4. Add environment SEARX_SETTINGS_TEMPLATE to .config.sh

5. Use environment $SEARX_SETTINGS_TEMPLATE in the utils/searx.sh script

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>