Developer documentation: Creating a template

Hello @dbeltran and @bdepaula,

Autosubmit Version

tag:v4.0.0b2

Summary

We lack content for developers in the documentation. We need at least a very simple example of creating templates in Autosubmit.

Besides, in https://autosubmit.readthedocs.io/en/master/introduction/index.html the Developing an EC-Earth Project section is duplicated (Quick Start and Developer Guide). I would remove it entirely and only have a first section in Developer Guide about how to add a simple template.