Resolve "Make out_path check in BasePlotCreator optional"

This MR adds the possibility to overwrite existing plots, where previously this would have led to an error that could not be recovered from.

This is achieved by adding an exist_ok argument that is parsed from the plot configuration. Also, it adds the documentation to describe how the overwriting can be achieved.

Furthermore, this takes care of some documentation issues on the way... (sorry for mixing this up!)

Anything to double-check?

• Implementation ok?
• Documentation ok?

Can this MR be accepted?

• Implemented the changes
• Tests added, adjusted, and extended
• Checked code coverage on new and adjusted code
• Add doc/plotting/faq.rst to address plotting FAQs
• Pipeline passes without warnings
• Changelog entry added
• History cleaned up
• Tested by @daniellake in utopya
• Approved by @herdeanu

After merge, this should be 🍒 -picked into release/v0.11

Related issues

Closes #126 (closed)