discv5: add diagram svg and wrap code in processing instruction

Seems like a processing instruction is the best way to contain the
source text. An HTML comment can't be used because it clashes with the
mermaid syntax.

Author: fjl

discv5/discv5-theory.md

@@ -369,22 +369,24 @@ each message in the diagram below. We use these abbreviations:
 - `H(X)`: [handshake message packet] containing request `X`
 - `s(M)`: [session message packet] containing message `M`
 
-    ```mermaid
-        sequenceDiagram
-            participant Alice
-            participant Relay
-            participant Bob
-
-            Relay-->>Alice: s(NODES[Bob's ENR])
-            Alice->>Bob: m(nonce,FINDNODE)
-            Note left of Alice:Hole punched in Alice's NAT for Bob
-            Note left of Alice:FINDNODE timed out
-            Alice->>Relay: s(RELAYINIT[nonce])
-            Relay->>Bob: s(RELAYMSG[nonce])
-            Bob-->>Alice: whoareyou(nonce)
-            Note right of Bob: Hole punched in Bob's NAT for Alice
-            Alice-->>Bob: H(FINDNODE)
-    ```
+<?mermaid
+    sequenceDiagram
+        participant Alice
+        participant Relay
+        participant Bob
+
+        Relay-->>Alice: s(NODES[Bob's ENR])
+        Alice->>Bob: m(nonce,FINDNODE)
+        Note left of Alice:Hole punched in Alice's NAT for Bob
+        Note left of Alice:FINDNODE timed out
+        Alice->>Relay: s(RELAYINIT[nonce])
+        Relay->>Bob: s(RELAYMSG[nonce])
+        Bob-->>Alice: whoareyou(nonce)
+        Note right of Bob: Hole punched in Bob's NAT for Alice
+        Alice-->>Bob: H(FINDNODE)
+?>
+
+![Diagram](./img/nat-hole-punching-flow.svg)
 
 Preconditions: Bob is behind NAT. Bob is contained in Relay's node table, they have an
 established session and Bob has sent a packet to Relay in the last ~20 seconds hence Relay