The animated Gifs you find in the DTail docs were created using:
On Fedora Linux 35.
On Fedora Linux 35 install
% sudo dnf install -y asciinema
asciicast2gif (for simplicity, the Docker image was used):
% docker pull asciinema/asciicast2gif
This of course assumes that Docker is up and running on your machine (out of scope for this documentation).
This is as simple as running
% asciinema rec recording.json
This will launch a sub-shell to be recorded. Once done, exit the sub-shell with
% docker run --rm -v $PWD:/data asciinema/asciicast2gif -t tango -s 2 recording.json recording.gif