[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>

searx/settings.yml

@@ -196,6 +196,9 @@ outgoing:
 #   - 'Open Access DOI rewrite'
 #   - 'Vim-like hotkeys'
 #   - 'Tor check plugin'
+#   # Read the docs before activate: auto-detection of the language could be
+#   # detrimental to users expectations / users can activate the plugin in the
+#   # preferences if they want.
 #   - 'Autodetect search language'
 
 # Configuration of the "Hostname replace" plugin: