Fix static HTML documentation

Since hugo v0.123.0, HTML files in the content/ directory are processed as a resource:

https://github.com/gohugoio/hugoDocs/issues/2428

Move the generated documentation to static/ to avoid this. Also, adapt the import script.

This is the result of:

  mkdir -p static/docs/api/
  mkdir -p static/docs/libnm/
  git mv content/docs/api/1.*      static/docs/api/
  git mv content/docs/api/latest   static/docs/api/
  git mv content/docs/libnm/1.*    static/docs/libnm/
  git mv content/docs/libnm/latest static/docs/libnm/
  sed -i 's|content/docs|static/docs|g' scripts/import-docs.sh

Also, pin the hugo image version.