Migrate techdemos from PyMAPDL repository

[RobPasMue]

We should try to migrate the techdemos being held in the PyMAPDL repository.

For now, let's try to keep it simple:

• Move the tech demos (as they are) to this repository.
• Set up a simple landing page which summarizes what are tech demos, which ones are available, etc.
• Leave them static - they will not update for now. This will be solved in Set up workflows for rendering techdemos #9.

These tasks will involve creating a simple landing page (right now there is already one, but you will have to adapt it as per point 2). Right now the landing page looks like this: https://techdemos.mapdl.docs.pyansys.com

[RobPasMue]

@clatapie start working on this whenever you have time =)