docs: README, London example, corrected Queensland figure, performance tables

[petercorke]

A review and extension of the README.

Corrections

• The Queensland figure was wrong. roads.png was drawn from places.json, whose coordinates are in metres while the road distances are in km, so the A* heuristic was about 1,000 times too large and the route it found, 1,759 km via Rockhampton, was not the shortest. queensland.json (the same routes and distances, coordinates in km) gives 1,659 km via Roma, which is what the notebook and the snippet compute. The figure is regenerated from the snippet, axes in km.
• The snippet now reads the same data file as the notebook it links to.
• g.component(v) was documented as "the component that vertex v belongs to"; it takes a component number. v.label is the component of a vertex.
• intro.rst: "pgaph-python" in the history paragraph.

Added

• An Installation section (pip install pgraph-python, Python 3.9 or later).
• A bigger example: routing through central London (the figure, the two routes and why they differ, a link to the notebook, the OpenStreetMap attribution).
• The method lists now include path_UCS, closest, near, within, subgraph, coordinates, build_kdtree, metric/SE2Metric, copy, export and validate; vertex_class and edge_class in the subclassing section; a runnable subclassing snippet.
• Performance: tables of the key operations for undirected and directed graphs at 100, 1,000 and 10,000 vertices, with the growth exponent, measured with examples/benchmark.py on current main.
• An outline: ## Installation, ## Examples, ## Using pgraph, ## Performance, ## History.
• Descriptive alternative text for the figures.

Docs only, apart from the regenerated PNG and the new docs/figs/london_route.png.

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✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.26%. Comparing base (cdbc54a) to head (d555170).

Additional details and impacted files

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  Files           3        3           
  Lines        1570     1570           
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  Hits         1527     1527           
  Misses         43       43           

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