Write a documentation for every submodule of the import_cmip6_data module.

Issue

I want to improve the README.md of the import_cmip6_data_module modules.

Goals

• Realize task #7 (closed).
• Realize task #8 (closed).
• Realize task #10 (closed).
• Realize task #37 (closed).

Criteria for each documentation

• The documentation holds an introduction explaining what the module/submodule does.
• The documentation states what are the different function and how they interact together : linearly or not ?
• The documentation states what the input you feed the module/submodule and the expected output.