Set up workflows for rendering techdemos

[RobPasMue]

One of the key points of having the tech demos separated from the rest of the PyMAPDL build is to alleviate the load of the PyMAPDL workflow. By not having to render these techdemos, workflow times will be cut.

However, this also implies setting up a workflow which runst the techdemos, updates them and checks that everything is running fine. We will have to:

• Set up a nightly workflow that runs all of them every day
• Set up a simple CI/CD workflow that ensures that changes performed are not braking existing code

Once the workflow is set up we will be able to ensure that our code is not broken

[RobPasMue]

@clatapie we will be doing this one together so that you can learn the workflow good practices we typically implement. Assigning it to both of us.