docs: add Randomness page explaining seed semantics

[leotrs]

Summary

Adds a new `randomness.rst` page to the User Guides documenting how XGI's `seed` arguments behave. Per Max's suggestion on PR #724.

The page covers:

• Accepted types: `None`, `int`, `numpy.random.Generator`
• The four reproducibility cases (incl. the surprising one: `seed=42` ≠ `seed=default_rng(42)`)
• Why and when to pass a single `Generator` to a multi-step pipeline
• Links to NumPy's upstream rng docs

Also wires it into the User Guides toctree and adds a card for discoverability.

Test plan

• Docs-only change, full test suite still passes (418 passed, 6 skipped)
• No tests added (docs page)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.61%. Comparing base (9e1e9af) to head (c8956bd).
⚠️ Report is 44 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #737      +/-   ##
==========================================
+ Coverage   93.65%   94.61%   +0.95%     
==========================================
  Files          66       66              
  Lines        5168     5271     +103     
==========================================
+ Hits         4840     4987     +147     
+ Misses        328      284      -44     

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