docs(reuse): add User Guide page on reusing functions and nodes (#1045)

[Hore01]

This adds a User Guide page that pulls together Hamilton's four patterns for reusing the same logic across different data sources, regions, or runtime contexts:

• driving the same function module from multiple Drivers
  (examples/feature_engineering/feature_engineering_multiple_contexts/)
• Builder.allow_module_overrides() for swapping same-named functions (examples/module_overrides/)
• @subdag to repeat a transformation graph inside one Driver (examples/reusing_functions/)
• @parameterized_subdag for many subdags at once, with the readability warning quoted from the source docstring about preferring static DAGs when you don't have many copies

Closes #1045.

The new page is docs/how-tos/reuse-nodes.rst, slotted into the User Guide toctree just after use-for-feature-engineering. Every code block on the page is a .. literalinclude:: pointing at the example
files that already exist in the repo (module_a.py, module_b.py, run.py, unique_users.py, reusable_subdags.py), so the prose can't drift away from the runnable code.

I tested locally on Python 3.12: sphinx (-b dirhtml) builds cleanly and no warnings reference the new page or any of the example files it pulls from. The 41 warnings I do see locally are the same autodoc warnings about optional plugin deps that already exist on main and that CI resolves via the full docs dependency group.

A couple of notes:

• The page deliberately doesn't introduce a new example folder. Per the issue, the existing reusing_functions/, module_overrides/ and feature_engineering_multiple_contexts/ examples already cover the patterns; the goal here is to point readers at them in one place with the right framing.
• Commit is DCO-signed.

[skrawcz]

thanks!