[cranelift docs] Fix grammar code blocks and heap image (#1339)

* [cranelift docs] Move grammar segments in IR docs to code blocks

* [cranelift docs] Replace dot code in IR docs with generated image.

This is how the image was generated

$ cat > heap.dot << EOF
digraph {
        node [
              shape=record,
              fontsize=10,
              fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans"
              ]
        "static" [label="mapped\npages|unmapped\npages|offset_guard\npages"]
}
EOF

$ dot -Tsvg -O heap.dot

* [cranelift docs] Fix indentation in grammar block

Co-Authored-By: bjorn3 <bjorn3@users.noreply.github.com>

Co-authored-by: bjorn3 <bjorn3@users.noreply.github.com>

cranelift/docs/heap.dot

@@ -0,0 +1,8 @@
+digraph {
+        node [
+              shape=record,
+              fontsize=10,
+              fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans"
+        ]
+        "static" [label="mapped\npages|unmapped\npages|offset_guard\npages"]
+}