Enhancement: write a document on how to use the HTML extension with a custom validator.nu running in docker

Created by: mgueury

See: https://hub.docker.com/r/sitevalidator/nu-cat/

To do

1. add a preference to set the server where the online validation happen
2. write some documentation on how to do this with the above docker container

Probably something as simple as this:

docker run -p 8888:8080 nu-cat You can find the validator at http://localhost:8888