Add a guide to use Thunderbird through Tor

1 job for guide/use_thunderbird_through_tor in 48 seconds
Name Stage Failure
failed
docker_build Build
Configure a credential helper to remove this warning. See
https://docs.docker.com/engine/reference/commandline/login/#credentials-store

Login Succeeded
$ docker build --build-arg ci_token=$CI_JOB_TOKEN --pull -t $IMAGE_TAG .
invalid argument "registry.git.autistici.org/ai/website:guide/use_thunderbird_through_tor" for "-t, --tag" flag: invalid reference format
See 'docker build --help'.
Cleaning up file based variables
ERROR: Job failed: exit code 125