[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>

Dockerfile

@@ -36,6 +36,7 @@ RUN apk add --no-cache -t build-dependencies \
     su-exec \
     python3 \
     py3-pip \
+    py3-numpy \
     libxml2 \
     libxslt \
     openssl \
@@ -43,6 +44,8 @@ RUN apk add --no-cache -t build-dependencies \
     uwsgi \
     uwsgi-python3 \
     brotli \
+ && pip3 install --no-cache setuptools wheel \
+ && sed -i s/fasttext-wheel/fasttext/ requirements.txt \
  && pip3 install --no-cache -r requirements.txt \
  && apk del build-dependencies \
  && rm -rf /root/.cache