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/results.json

@@ -19,35 +19,35 @@
     "d": 9.5,
     "fc": 3.0,
     "fy": 60.0,
-    "As": 0.613592,
-    "a": 1.804683,
-    "et": 0.010423,
-    "phi": 0.9,
-    "Mn": 26.377285,
-    "phiMn": 23.739557,
+    "As": 3.141593,
+    "a": 9.239978,
+    "et": -0.000378,
+    "phi": 0.65,
+    "Mn": 76.65503,
+    "phiMn": 49.82577,
     "As_min": 0.253333,
     "Vc": 8.325383,
     "phiVc": 6.244037,
     "cover": 1.5,
     "stirrup_size": 3,
-    "stirrup_spacing": 6.0,
+    "stirrup_spacing": 8.0,
     "stirrup_db": 0.375,
-    "bar_size": 5,
-    "n_bars": 2,
-    "bar_db": 0.625,
-    "top_bar_size": 4,
+    "bar_size": 8,
+    "n_bars": 4,
+    "bar_db": 1.0,
+    "top_bar_size": 8,
     "n_top_bars": 2,
-    "top_bar_db": 0.5
+    "top_bar_db": 1.0
   },
   "checks": {
     "flexure": {
       "demand": 23.44,
-      "capacity": 23.739557,
+      "capacity": 49.82577,
       "ok": true
     },
     "minimum_steel": {
       "demand": 0.253333,
-      "capacity": 0.613592,
+      "capacity": 3.141593,
       "ok": true
     },
     "shear": {