Fix broken autoclass:: references on matrices/planning doc pages

Both docs/source/matrices.rst and docs/source/planning.rst referenced
pgraph.PGraph.PGraph, a class that no longer exists post-rename (it's
now _BaseGraph) -- and in fact never existed at that dotted path even
before the rename, since the src/pgraph/ layout migration changed the
real import path without these two files being updated to match (the
same bug already fixed for every other doc page in the type-hints
pass, but missed here).

autodoc silently failed to import the class on both pages, so
"Convert graph to matrix form" and "Path planning" rendered as bare
headings with zero API content -- in both the HTML and PDF (verified
with a real latexmk run) builds. Caught during final pre-1.0.0 review
of a built PDF.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: petercorke

docs/source/matrices.rst

@@ -3,5 +3,5 @@ Convert graph to matrix form
 
 These methods convert a graph to a matrix representation .
 
-.. autoclass:: pgraph.PGraph.PGraph
+.. autoclass:: pgraph.PGraph._BaseGraph
     :members: distance, adjacency, degree, incidence, Laplacian

docs/source/planning.rst

@@ -3,5 +3,5 @@ Path planning
 
 These methods find optimal paths from a start vertex to a goal vertex.
 
-.. autoclass:: pgraph.PGraph.PGraph
+.. autoclass:: pgraph.PGraph._BaseGraph
    :members: path_BFS, path_UCS, path_Astar