[MNT]: Analyse, define and document Artist.axes #31118

Summary

The situation is currently mixed and not well-defined, e.g.

• matplotlib/lib/matplotlib/artist.py

  Lines 215 to 218 in 45caf0e

  	try:
  	self.axes = None
  	except AttributeError:
  	# Handle self.axes as a read-only property, as in Figure.

  
  does always raise AFAICS should not be there, because Artist has an Axes property.
• Semantics: Loosely speaking Artist. axes is the Axes the Artist is in. Do we need to specify this explicitly? Who has the authority to set this? Likely only Axes.add_artist?
• Semantics 2: Figure redefines the property to "The list of Axes in the figure"
• Some Artist subclasses define property setters
• Should Artist.axes be public at all or is it an implementation detail.
• we should have one place where we document the logic behind all this. Could e.g. be with the property.
• probably more ...

Proposed fix

No response