Fix/improve documentation

[PhilippRue]

Make readthedocs documentation work and improve the documentation of the package's features

Until this is merged the docs for this branch are availabe under: https://aiida-jutools.readthedocs.io/en/fix_docs/

[PhilippRue]

So far the readthedocs docu does not give the auto generated API reference, to build this locally go to the docs directory and run make html where the API reference is included.

TODO: A comparison to the docu settings of aiida-kkr or aiida-fleur should allow us to fix this issue.

[Irratzo]

With regard to this pull request, I'm rebasing the project config files on the aiida-plugin-cutter template. Motivation: 1) have config up to date, no outdated stuff, 2) have AiiDA-integrated testing set up correctly. This is happening in the branch fix_docs_cookiecutter. Will finish this in June-July.

• rebase project config files on aiida-plugin-cutter template
• adjust config files until everything works (setup, Actions, docs, ..., optional: PyPI Action)
• merge into fix_docs branch
• fix docs: base structure skeleton on hitchhiker, implemented example: DScribe docs (all sections empty).
• fix docs: complete API section (autodoc, missing class docstrings, etc.), until RTD page looks good.
• (take care that ruess' earlier refactoring of roman's test files are not lost)
• merge into develop branch (optional: merge develop into master branch)
• close pull request