Update concrete-beam example, section placement, and README

- Longitudinal bars 4 #8 bottom / 2 #8 top; stirrups #3 @ 8 in.
- Move the section figure after Loads and Beam Demand.
- Add concrete-beam/README.md.
- Regenerate results.json, assets/beam-section.typ, and the PDF.

Note: the example-value tests (test_example_flexure,
test_reinforcement_geometry) are not updated for the new input and
currently fail.

concrete-beam/concrete-beam.typ

@@ -38,13 +38,6 @@ Simple-span rectangular beam under uniform gravity load. Numbers come from `calc
   caption: [#n.span ft simply supported beam, #n.bw in × #n.h in rectangular section.],
 )
 
-== Section and Reinforcement
-
-#align(right, block(width: 2.5in, figure(
-  beam-section(),
-  caption: [Beam cross-section],
-)))
-
 == Loads and Beam Demand
 
 #calcline([$L = #n.span " ft"$], [Simple span])
@@ -65,6 +58,13 @@ Simple-span rectangular beam under uniform gravity load. Numbers come from `calc
   [Support shear],
 )
 
+== Section and Reinforcement
+
+#align(right, block(width: 2.5in, figure(
+  beam-section(),
+  caption: [Beam cross-section],
+)))
+
 == Flexural Strength
 
 #calcline([$b_w = #n.bw " in"$], [Beam width])