[docs] Switch from searx to SearXNG

Closes: https://github.com/searxng/searxng/issues/450
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/admin/index.rst

@@ -12,6 +12,7 @@ Administrator documentation
    installation-nginx
    installation-apache
    installation-docker
+   installation-switch2ng
    update-searxng
    engines/index
    api

docs/admin/installation-switch2ng.rst

@@ -0,0 +1,75 @@
+.. _installation switch2ng:
+
+============================
+Switch from searx to SearXNG
+============================
+
+.. sidebar:: info
+
+   - :pull:`456`
+   - `A comment about rolling release <https://github.com/searxng/searxng/pull/446#issuecomment-954730358>`_
+
+.. contents:: Contents
+   :depth: 2
+   :local:
+   :backlinks: entry
+
+If you have a searx installation on your sever and want to switch to SearXNG,
+you need to uninstall searx first.  If you have an old searx docker installation
+replace your docker image / see :ref:`installation docker`.
+
+If your searx instance was installed *"Step by step"* or by the *"Installation
+scripts"*, you need to undo the installation procedure completely.  If you have
+morty & filtron installed, it is recommended to uninstall these services also.
+In case of scripts, to uninstall use the scripts from the origin you installed
+searx from.
+
+If you have removed the old searx installation, clone from SearXNG and and start
+with your installation procedure (e.g. :ref:`installation scripts`):
+
+.. code:: bash
+
+   $ cd ~/Downloads
+   $ git clone https://github.com/searxng/searxng.git searxng
+   $ cd searxng
+   $ ...
+
+``.config.sh``
+==============
+
+Please take into account; SearXNG has normalized ``.config.sh`` with
+``settings.yml`` and some of the environment settings has been removed from or
+renamed in the ``.config.sh``:
+
+- :patch:`[mod] normalize .config.sh with settings.yml <f61c918d>`
+- :patch:`[fix] ./utils/filtron.sh - FILTRON_TARGET from YAML settings <7196a9b5>`
+- :patch:`SearXNG: SEARXNG_SETTINGS_PATH <253b8503>`
+
+
+Check after Installation
+========================
+
+Once you have done your installation, you can run a SearXNG *check* procedure,
+to see if there are some left overs.  In this example there exists a *old*
+``/etc/searx/settings.yml``::
+
+   $ sudo -H ./utils/searx.sh install check
+
+   ============================
+   SearXNG (check installation)
+   ============================
+   ERROR: settings.yml in /etc/searx/ is deprecated, move file to folder /etc/searxng/
+   INFO:  SearXNG instance already installed at: /usr/local/searx/searx-src
+   ...
+   INFO:  Service account searx exists.
+   INFO:  ~searx: python environment is available.
+   INFO:  ~searx: SearXNG software is installed.
+   INFO:  uWSGI app searxng.ini is enabled.
+   INFO    searx                         : merge the default settings ( /usr/local/searx/searx-src/searx/settings.yml ) and the user setttings ( /etc/searxng/settings.yml )
+   INFO    searx                         : max_request_timeout=None
+
+
+To *check* the filtron & morty installations, use similar commands::
+
+  $ sudo -H /utils/filtron.sh install check
+  $ sudo -H /utils/morty.sh   install check

docs/admin/installation.rst

@@ -4,6 +4,10 @@
 Installation
 ============
 
+.. sidebar:: info
+
+   :ref:`installation switch2ng`
+
 *You're spoilt for choice*, choose your preferred method of installation.
 
 - :ref:`installation docker`