Refine beam section figure and derive As from bars

- Derive As from the provided longitudinal bars instead of taking it as
  input; drop the redundant As_bars output and update tests.
- Draw a fixed schematic 1 in cover and pin the bottom bars to the
  stirrup's bottom leg so the figure no longer depends on cover or d.
- Fixed stirrup stroke with a rounded 3 pt corner radius.
- Right-align the figure, constrain its caption to the image width, and
  shorten the caption to "Beam cross-section".
- Document input.yaml fields.

concrete-beam/results.json

@@ -19,12 +19,12 @@
     "d": 9.5,
     "fc": 3.0,
     "fy": 60.0,
-    "As": 0.62,
-    "a": 1.823529,
-    "et": 0.010285,
+    "As": 0.613592,
+    "a": 1.804683,
+    "et": 0.010423,
     "phi": 0.9,
-    "Mn": 26.623529,
-    "phiMn": 23.961176,
+    "Mn": 26.377285,
+    "phiMn": 23.739557,
     "As_min": 0.253333,
     "Vc": 8.325383,
     "phiVc": 6.244037,
@@ -37,18 +37,17 @@
     "bar_db": 0.625,
     "top_bar_size": 4,
     "n_top_bars": 2,
-    "top_bar_db": 0.5,
-    "As_bars": 0.613592
+    "top_bar_db": 0.5
   },
   "checks": {
     "flexure": {
       "demand": 23.44,
-      "capacity": 23.961176,
+      "capacity": 23.739557,
       "ok": true
     },
     "minimum_steel": {
       "demand": 0.253333,
-      "capacity": 0.62,
+      "capacity": 0.613592,
       "ok": true
     },
     "shear": {