move tree generation methods to src/sage/graphs/generators/trees.pyx

[dcoudert]

Fixes #39180 .

This PR implements the changes proposed in #39180 (comment):

• move src/sage/graphs/trees.pyx and src/sage/graphs/trees.pxd to src/sage/graphs/generators/.
• move to this file all methods for generating trees, including graphs.trees(), graphs.nauty_gentreeg(), graphs.BalancedTree(), graphs.FibonacciTree(), graphs.RandomTree(), graphs.RandomTreePowerlaw(), graphs.RandomLobster(), etc.
• add some documentation and examples showing the different ways to generate trees at the top of module trees.pyx.
• ensure that all internal links to moved methods are properly updated.

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