docs: pack the L5 diagram tighter (ranksep 1.2 -> 0.4)

ranksep=1.2 over-stretched it vertically (1193x1483, lots of whitespace).
Drop to 0.4 + nodesep 0.25: 1202x1063, ~28% less area, no vertical sprawl,
stays roughly square. Width is floored by the widest node label; click-to-
zoom (zoom.js) covers reading detail.

docs/_source/google-l5-rag-10m.rst

@@ -33,7 +33,7 @@ Pipeline
 
    digraph arborist_rag {
      rankdir=TB;
-     ranksep=1.2; nodesep=0.2;
+     ranksep=0.4; nodesep=0.25;
      fontname="Helvetica";
      node [fontname="Helvetica", shape=box, style=rounded, fontsize=10];
      edge [fontname="Helvetica", fontsize=9];