gh-35266: `sage.graphs`: Add `# optional` doctest tags for modularization
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### 📚 Description
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These tags make it possible/meaningful to doctest `sage.graphs` even
when some "standard" packages (`networkx`, `numpy`, the modularized
distribution packaging providing `sage.groups`) are not installed.
- Part of #29705
- Split out from #34998
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### ⌛ Dependencies
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This is just one commit on top of:
- #35237
URL: https://github.com/sagemath/sage/pull/35266
Reported by: Matthias Köppe
Reviewer(s): David Coudert, Dima Pasechnik, Matthias Köppe
