chore(spec): Clean up non-local edges CFG example (#1369)

Closes #248 

There are still some visual glitches, but it should be more readable
now.

Adds two invisible order edges to the input nodes, changing this
```mermaid
flowchart
    subgraph CFG
        direction TB
        subgraph Entry
            direction TB
            EntryIn["Input"] -- "P" --> Entry_["P?1"]
            Entry_ -- "[()|(P)]" --> EntryOut["Output"]
            F
        end
        subgraph BB1
            direction TB
            BB1In["Input"]
            BB1_["Const"] -- "[()]" --> BB1Out["Output"]
            G -- "angle" --> BB1Out
        end
        subgraph BB2
            direction TB
            BB2In["Input"] -- "P" --> H -- "P" --> BB2_["P?2"]
            BB2_ -- "[()|()]" --> BB2Out["Output"]
        end
        subgraph BB3
            direction TB
            BB3In["Input"]
            BB3_["Const"] -- "[()]" --> BB3Out["Output"]
            C -- "angle" --> BB3Out
        end
        subgraph Exit
        end
        Entry -- "0" --> BB1
        Entry -- "1" --> BB2
        BB2 -- "0" --> BB1
        BB2 -- "1" --> BB3
        BB1 -- "0" --> Exit
        BB3 -- "0" --> Exit
    end
    A -- "P" --> CFG
    A -- "qubit" --> Rz_out["Rz"]
    CFG -- "angle" --> Rz_out
    Rz_out -- "qubit" --> B
    A == "angle" ==> F
    F == "angle" ==> G
    linkStyle 12,13,14,15,16,17 stroke:#ff3,stroke-width:4px;
```
into this
```mermaid
flowchart
    subgraph CFG
        direction TB
        subgraph Entry
            direction TB
            EntryIn["Input"] -- "P" --> Entry_["P?1"]
            Entry_ -- "[()|(P)]" --> EntryOut["Output"]
            F
        end
        subgraph BB1
            direction TB
            BB1In["Input"]
            BB1_["Const"] -- "[()]" --> BB1Out["Output"]
            G -- "angle" --> BB1Out
            BB1In ~~~ G
        end
        subgraph BB2
            direction TB
            BB2In["Input"] -- "P" --> H -- "P" --> BB2_["P?2"]
            BB2_ -- "[()|()]" --> BB2Out["Output"]
        end
        subgraph BB3
            direction TB
            BB3In["Input"]
            BB3_["Const"] -- "[()]" --> BB3Out["Output"]
            C -- "angle" --> BB3Out
            BB3In ~~~ C
        end
        subgraph Exit
        end
        Entry -- "0" --> BB1
        Entry -- "1" --> BB2
        BB2 -- "0" --> BB1
        BB2 -- "1" --> BB3
        BB1 -- "0" --> Exit
        BB3 -- "0" --> Exit
    end
    A -- "P" --> CFG
    A -- "qubit" --> Rz_out["Rz"]
    CFG -- "angle" --> Rz_out
    Rz_out -- "qubit" --> B
    A == "angle" ==> F
    F == "angle" ==> G
    linkStyle 12,13,14,15,16,17 stroke:#ff3,stroke-width:4px;
```

specification/hugr.md

@@ -673,6 +673,7 @@ flowchart
             BB1In["Input"]
             BB1_["Const"] -- "[()]" --> BB1Out["Output"]
             G -- "angle" --> BB1Out
+            BB1In ~~~ G
         end
         subgraph BB2
             direction TB
@@ -684,6 +685,7 @@ flowchart
             BB3In["Input"]
             BB3_["Const"] -- "[()]" --> BB3Out["Output"]
             C -- "angle" --> BB3Out
+            BB3In ~~~ C
         end
         subgraph Exit
         end