[enh] CI - add shell script formatter

Implement rules and functions to format shell scripts:

    $ make format.shell

or alternatively to format all source code:

    $ make format

The formatter `shfmt` reads the rules from the editorconfig[1]

  If any EditorConfig files are found, they will be used to apply formatting
  options.  If any parser or printer flags are given to the tool, no
  EditorConfig files will be used.

[1] https://github.com/patrickvane/shfmt?tab=readme-ov-file#description

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

docs/dev/makefile.rst

@@ -188,24 +188,22 @@ sources of the theme need to be rebuild.  You can do that by running::
 
     $ LIVE_THEME=simple make run
 
-.. _make format.python:
+.. _make format:
 
-``make format.python``
+``make format``
 ======================
 
-Format Python source code using `Black code style`_.  See ``$BLACK_OPTIONS``
-and ``$BLACK_TARGETS`` in :origin:`Makefile`.
-
-.. attention::
-
-   We stuck at Black 22.12.0, please read comment in PR `Bump black from 22.12.0
-   to 23.1.0`_
-
-.. _Bump black from 22.12.0 to 23.1.0:
-   https://github.com/searxng/searxng/pull/2159#pullrequestreview-1284094735
-
 .. _Black code style:
    https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html
+.. _shfmt: https://github.com/mvdan/sh?tab=readme-ov-file#shfmt
+.. _EditorConfig: https://github.com/patrickvane/shfmt?tab=readme-ov-file#description
+
+- Format Python source code using `Black code style`_.  See ``$BLACK_OPTIONS``
+  and ``$BLACK_TARGETS`` in :origin:`Makefile`.
+
+- Format Shell scripts using shfmt_. The formatter ``shfmt`` reads the rules
+  from the EditorConfig_ files.
+
 
 .. _make clean:
 

docs/dev/quickstart.rst

@@ -27,7 +27,7 @@ Here is how a minimal workflow looks like:
 
 1. *start* hacking
 2. *run* your code: :ref:`make run`
-3. *format & test* your code: :ref:`make format.python` and :ref:`make test`
+3. *format & test* your code: :ref:`make format` and :ref:`make test`
 
 If you think at some point something fails, go back to *start*.  Otherwise,
 choose a meaningful commit message and we are happy to receive your pull