Link guides back to the Fatiando-wide pages

[leouieda]

Our CoC and Authorship, Contributing, and Maintainers Guides don't need to be repeated here since they are already in the fatiando/community repository. Remove the Authorship and Maintainers documents. Replace the content in CoC and Contributing documents with links to the organization wide documents.

Reminders:

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[leouieda]

@santisoler what do you think? We could propagate this to the other repos afterwards.