Add width and baseline properties documentation

[StefRe]

• add Width property class
• fix links for baseline and width properties (Area, Bar, Bars, Dash)

current documentation contains undefined links:

The baseline property just needed a link to Coordinate properties. For the width property I created a new IntervalProperty class and added the corresponding section to the properties tutorial:

I couldn't think of any meaningful test, so I'm afraid test coverage will slightly decrease.

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I noticed that the legend and normed class variables are defined without a leading underscore, but the Offset and FontSize classes use _legend:

seaborn/seaborn/_core/properties.py

Lines 317 to 325 in 16c025d

	class Offset(IntervalProperty):
	"""Offset for edge-aligned text, in point units."""
	_default_range = 0, 5
	_legend = False
	class FontSize(IntervalProperty):
	"""Font size for textual marks, in points."""
	_legend = False

I have stuck to the variant without underscore as defined in the base class in this PR.