[MNT]: Remove the userdemo section in examples #25800

Summary

https://matplotlib.org/devdocs/gallery/index.html#userdemo

This section was a collection of plots used via .. literalinclude:: in the user documentation.

It's existence was only motivated structurally. This does not fit with the semantic sectioning of the examples. Userdemo should thus be removed.

Proposed fix

If the examples are still used in an .. literalinclude::, they should be converted to a figure directive.

If they are not used anymore, they should either

• be deleted if they do not contain relevant new aspects (e.g. [Doc]: Delete examples made redundant by annotation tutorial rewrite (annotate_simple01, ...) #24723) or
• be relocated to other sections.