docs: what a vertex holds (edge lists, with example and figure)

[petercorke]

Adds a "What a vertex holds" part to the Edges & Vertices page, beside the data-structures diagram.

• The private lists of a vertex (_edgelist, _inlist) and its _graph back-pointer, and what each graph kind puts in them (an undirected loop is held twice).
• A worked example (parallel edges e1/e2, e3, loop e4), drawn for both UGraph and DGraph, with the resulting lists per vertex.
• How the public methods and degrees read those lists.

Every table row was checked against the source and against the example run on both graph kinds.

Docs only; the build has no warnings.

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.41%. Comparing base (b965468) to head (f0ad331).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #56   +/-   ##
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  Coverage   96.41%   96.41%           
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  Files           3        3           
  Lines        1339     1339           
=======================================
  Hits         1291     1291           
  Misses         48       48           

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