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/input.yaml

@@ -31,12 +31,12 @@ cover: "1.5 in"
 # Stirrup bar number (#3).
 stirrup_size: 3
 # Stirrup spacing along the span, on center.
-stirrup_spacing: "6 in"
+stirrup_spacing: "8 in"
 # Bottom (tension) longitudinal bar number (#5).
-bar_size: 5
+bar_size: 8
 # Number of bottom longitudinal bars.
-n_bars: 2
+n_bars: 4
 # Top (hanger) longitudinal bar number (#4).
-top_bar_size: 4
+top_bar_size: 8
 # Number of top hanger bars.
 n_top_bars: 2