Code review comment for ~athos-ribeiro/ubuntu-docker-images:update-templates-21.04-makefile

Thanks Athos! A few changes:

In the README:
- we can get rid of all the "Getting data from images repositories" instructions, as data?* files will now live in this repository.
- also, the .devcontainer could run the steps describe in the README for us :)

For the cassandra.yaml file:
- I think we need to change the version to remove the "rc" modifier as it wouldn't be part of the image tag. I also believe the 20.04 version is not an LTS, right?
- There's a "dead" link for the Kubernetes part, "TBD". This would need the absolute link to the proper configuration files.

With these changes this works well for me!

PS: I have a commit ready to fix the 3 first bullets but I don't know what to do with it. This is what my .devcontainer/Dockerfile looks like: (feel free to commit it yourself directly)
---
# See here for image contents: https://github.com/microsoft/vscode-dev-containers/tree/v0.148.1/containers/python-3/.devcontainer/base.Dockerfile
ARG VARIANT="3"
FROM mcr.microsoft.com/vscode/devcontainers/python:0-${VARIANT}

RUN apt-get update && export DEBIAN_FRONTEND=noninteractive \
    && apt-get -y install --no-install-recommends python3-mako python3-yaml

RUN cd /usr/bin && git clone https://github.com/misterw97/RenderDown \
    && pip3 --disable-pip-version-check --no-cache-dir install -r /usr/bin/RenderDown/requirements.txt \
    && rm -rf /tmp/pip-tmp
ENV RENDERDOWN /usr/bin/RenderDown/renderdown.py