[docs] update_external_bangs.py - implement script to document bangs

Function 'write_external_bang_doc(..)' generates documentation of the external
bangs in folder 'docs/admin/external_bang'.  Ducumentation will be update when
external bangs are updated::

    ./manage pyenv.cmd searx_extra/update/update_external_bangs.py

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

docs/admin/external_bang.rst

@@ -0,0 +1,19 @@
+.. _external bang:
+
+===============
+External !!bang
+===============
+
+.. warning::
+
+   *External Bangs* are shortcuts that quickly redirect you to search results on
+   other sites.
+
+     **The search is done on the external site without protecting your privacy.**
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Contents
+   :glob:
+
+   external_bang/*/index

docs/admin/index.rst

@@ -14,6 +14,7 @@ Administrator documentation
    installation-docker
    update-searx
    engines/index
+   external_bang
    api
    architecture
    filtron

docs/user/search_syntax.rst

@@ -9,13 +9,17 @@ Searx allows you to modify the default categories, engines and search language
 via the search query.
 
 Prefix ``!``
-  to set Category/engine
+  to set category or :ref:`engine <configured engines>`
+
+Prefix: ``!!``
+  to take you to search results on other sites (also know as :ref:`bang <external bang>`)
 
 Prefix: ``:``
   to set language
 
 Prefix: ``?``
-  to add engines and categories to the currently selected categories
+  to add :ref:`engines <configured engines>` and categories to the
+  currently selected categories
 
 Abbrevations of the engines and languages are also accepted.  Engine/category
 modifiers are chainable and inclusive (e.g. with :search:`!it !ddg !wp qwer
@@ -28,6 +32,10 @@ categories and languages.
 Examples
 ========
 
+*Bang* take you to search results on Wikipedia:
+
+- :search:`!!w gallileo <?q=%21%21w%20gallileo>`
+
 Search in wikipedia for ``qwer``:
 
 - :search:`!wp qwer <?q=%21wp%20qwer>` or