docs: make the L5 pipeline diagram portrait (ranksep=1.2)

TB alone left it landscape (1202x1101, h/w 0.92). Width is floored by the
widest node label (can't shrink without trimming labels), so bump
ranksep to stretch vertically: 1193x1483, h/w 1.24 — clearly top-down/
vertical for the docs column.

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

@@ -33,6 +33,7 @@ Pipeline
 
    digraph arborist_rag {
      rankdir=TB;
+     ranksep=1.2; nodesep=0.2;
      fontname="Helvetica";
      node [fontname="Helvetica", shape=box, style=rounded, fontsize=10];
      edge [fontname="Helvetica", fontsize=9];