[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"]
+}

cranelift/docs/ir.md

@@ -95,11 +95,12 @@ execution can never fall through to the next EBB without an explicit branch.
 
 A `.clif` file consists of a sequence of independent function definitions:
 
-.. productionlist::
-    function_list : { function }
-    function      : "function" function_name signature "{" preamble function_body "}"
-    preamble      : { preamble_decl }
-    function_body : { extended_basic_block }
+```
+function_list : { function }
+function      : "function" function_name signature "{" preamble function_body "}"
+preamble      : { preamble_decl }
+function_body : { extended_basic_block }
+```
 
 ### Static single assignment form
 
@@ -397,14 +398,15 @@ be known when the function call is compiled. The function signature describes
 how to call the function, including parameters, return values, and the calling
 convention:
 
-.. productionlist::
-    signature    : "(" [paramlist] ")" ["->" retlist] [call_conv]
-    paramlist    : param { "," param }
-    retlist      : paramlist
-    param        : type [paramext] [paramspecial]
-    paramext     : "uext" | "sext"
-    paramspecial : "sret" | "link" | "fp" | "csr" | "vmctx" | "sigid" | "stack_limit"
-    callconv     : "fast" | "cold" | "system_v" | "fastcall" | "baldrdash_system_v" | "baldrdash_windows"
+```
+signature    : "(" [paramlist] ")" ["->" retlist] [call_conv]
+paramlist    : param { "," param }
+retlist      : paramlist
+param        : type [paramext] [paramspecial]
+paramext     : "uext" | "sext"
+paramspecial : "sret" | "link" | "fp" | "csr" | "vmctx" | "sigid" | "stack_limit"
+callconv     : "fast" | "cold" | "system_v" | "fastcall" | "baldrdash_system_v" | "baldrdash_windows"
+```
 
 A function's calling convention determines exactly how arguments and return
 values are passed, and how stack frames are managed. Since all of these details
@@ -620,16 +622,7 @@ returns a pointer that is guaranteed to trap. Heap addresses can be smaller than
 the native pointer size, for example unsigned `i32` offsets on a 64-bit
 architecture.
 
-.. digraph:: static
-    :align: center
-    :caption: Heap address space layout
-
-    node [
-        shape=record,
-        fontsize=10,
-        fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans"
-    ]
-    "static" [label="mapped\npages|unmapped\npages|offset_guard\npages"]
+![Heap address space layout](./heap.svg)
 
 A heap appears as three consecutive ranges of address space: