doc: Pregel Tutorial - #809

…ney/build-upgrades

…ney/build-upgrades

Add a 'Choosing a Termination Condition' subsection to 'How Fast Does It
Converge?' in the Pregel tutorial: a table of the three conditions, the
cost of the two dynamic ones (each adds a Spark action per superstep),
and per-algorithm guidance for when each applies.

Also fix the user guide's vertex-voting bullet, which named
stopIfAllNonActiveVertices (the actual API is
setStopIfAllNonActiveVertices) and had an unclosed paren.

Switch generate_diagrams.py from the PhantomJS-based mmdc converter to
mermaidx, which bundles its own JS engine and needs no browser or Node
install. The new subgraph syntax separates the node id from the title, so
restore the G numbers in the graphlets overview titles ("G1 single directed
edge") that the bare-title syntax had provided implicitly.

Regenerate all motif and Pregel diagrams with the new renderer, and wire the
SVG versions into the motif tutorial in place of the older PNG figures.

Move the SparkSession setup, path resolution, and the repartition/checkpoint/
cache load of Nodes.parquet and Edges.parquet into 03-data-setup.md, which
now ends with nodes_df, edges_df, and g ready to use. The motif and Pregel
tutorials drop their duplicated copies of that boilerplate and state up front
that they continue from those objects, keeping only the imports each one adds.

Correct the prerequisites while here: Java 21 and Spark 4.1.3, matching the
versions the tutorials were actually written against.

Add a stat.corr call to the reputation propagation example and report the
result: authority and ViewCount correlate at only 0.07, so state that number
rather than implying the two usually track each other. Widen the truncate
width so the question titles are readable, and bold the point that joins grow
quadratically per hop while Pregel just needs a larger setMaxIter.

Replace the "the implementation would use..." hand-wave with a working
average-answer-score-per-tag algorithm, in both the tutorial and pregel.py.
It reverses the Tags edges so both hops travel with sendMsgToDst, sends a
struct carrying score and count together, and unpacks it with two
withVertexColumn definitions - which also demonstrates the vertex-vs-triplet
expression contexts.

Add two diagrams: one mapping each design question to its Pregel paper
concept and GraphFrames API call, one tracing scores from Answer to Question
to Tag. Note in the text that the four questions restate the components of
the original Pregel model, and add the vertex-centric literature behind that
framing to Further Reading - the McCune survey, Tian et al. on "think like a
vertex", and Colyer's walkthrough of the Pregel paper.

Switch figcaptions to white for legibility on the dark docs theme.

…ce,.

…rial assets in black and isort. Add mispelling exceptions.