Create a general template for the packages

[gareth-j]

We need a general template fo the packages. We should have

For each of the packages

• Introduction
• User Guide
• Command Line Interface
• API Reference
• Contributions
• FAQ

In the higher level documentation:

• Overview
• User Guide
• Installing
• Contributing

[corentincarton]

After the docathon, we still need to do the following:

• Enforce the structure above in all the packages
• Review the entire "Contributing" section
• Review the landing page of the main documentation and reorganise sections
• Reference the "Contributing" section from the main anemoi documentation in the anemoi packages, see anemoi-core/training as an example.
• Add a code of conduct, we can simply link to https://docs.github.com/en/site-policy/github-terms/github-community-code-of-conduct
• Review/update/add all the links

@VeraChristina to lead this task, tagging @aaron-hopkinson to lead the user guide on the main documentation.

[anaprietonem]

About the 'Enforce the structure above in all the packages', if we could have a cookiecutter template repo that people could pull from when doing new docs. It doesn't address the aspect about enforcing it on current packages