[docs] move searxng_extra/ stuff to the developer section.

Stuff in folder searxng_extra/ is not suitable for normal users and should only
be used by developers.

The script searxng_extra/standalone_searx.py must not give the impression that
it improves privacy. [1]

[1] https://github.com/searxng/searxng/pull/651#issuecomment-1001389726

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

docs/dev/index.rst

@@ -16,3 +16,4 @@ Developer documentation
    lxcdev
    makefile
    reST
+   searxng_extra/index

docs/dev/searxng_extra/index.rst

@@ -5,7 +5,7 @@ Tooling box ``searxng_extra`` for developers and users
 ======================================================
 
 In the folder :origin:`searxng_extra/` we maintain some tools useful for
-developers and users.
+developers.
 
 .. toctree::
    :maxdepth: 2

docs/index.rst

@@ -33,7 +33,6 @@ If you don't trust anyone, you can set up your own, see :ref:`installation`.
    user/index
    admin/index
    dev/index
-   searxng_extra/index
    utils/index
    src/index