[mod] improve 'Autodetect search language' plugin

- Add documentation to the plugin
- Harmonize FastText language model with SearXNG's language model

Reosurces::

    import fasttext                                    # --> +10 MB
    fasttext.load_model(str(data_dir / 'lid.176.ftz')) # --> +4MB

Suggested-by: @dalf

- To speed up and simplify the deployment use fasttext-wheel instead of fasttext
- Building numpy on the Alpine Linux of docker-images takes ages --> install
  py3-numpy from Alpines package manager (apk)
- Alpine Linux on docker-images (musl libc) do not support fasttext-wheel (gnu
  libc) --> patch Dockerfile and build from fastetxt:

     sed -i s/fasttext-wheel/fasttext/ requirements.txt

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

requirements.txt

@@ -16,4 +16,4 @@ setproctitle==1.3.2
 redis==4.4.0
 markdown-it-py==2.1.0
 typing_extensions==4.4.0
-fasttext==0.9.2
+fasttext-wheel==0.9.2