Update of the data request text / tutorial

[jesusff]

Currently, the explanation of the data request is in the README file shown as the front page in this repository. We should consider (see Section 5.5 in the TF-CMIP7 report):

• where to maintain it (keep it in the README? Move it to an mkdocs website which can be converted to a publishable PDF including the description and the list of variables?)
• whether to update the Core/Tier1/Tier2 concept for CORDEX-CMIP7