Revise docstrings

[gplssm]

Get all docstrings right

• Correct sections according to style guide
• ...

[gplssm]

This is now the parent issue for the revision of docstring. @miguelparadacontzen takes care about structure, completeness, and formatting of docstrings and provides as much information as possible. Remaining tasks to complete docstrings will be assigned to @nesnoj and @gplssm.

Let's therefore create individual issues per file or module and keep track of them along the milestone.

[gplssm]

In order to use work by @miguelparadacontzen irregardless of unfinished polishing of docstrings (i.e. #202) the branch features/clean-documentation will be merged into dev. This brings a properly building API/class documentation.
The merge had a bunch of conflicts (~30 files).

• Remove errors/warnings during sphinx-build
• Proof that Ding0 is running properly
• If needed, recover removed files (i.e. pypsa_io and some .rst files)
• Write "What's New" entry
• Close docs related issues
• Merge to dev

[gplssm]

After renaming, ding0 runs properly. Integrity of resulting data in terms of being identical to dev branch was tested successfully on grid ids 11,45,54.

This issue will be closed because API docs build now and the long living branch features/clean-documentation is merged. There is still correction and addition of/to the docs needed as issues of the docs milestone describe these needs.