doc: fix ASCII diagram alignment in user index.

Replace `◀` / `▶` (often rendered as double-width emoji glyphs) with
`←` / `→` (single-cell in every monospace font) so the box borders
line up correctly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

doc/user/index.md

@@ -17,7 +17,7 @@ session as a script.
 ```
 ┌──────────┐    ┌──────────┐
 │ Module A │    │ Module B │
-│  Part J1 │◀──▶│  Part P1 │   <— a Connection (built by `connect`)
+│  Part J1 │←──→│  Part P1 │   <— a Connection (built by `connect`)
 │   pins   │    │   pins   │
 └──────────┘    └──────────┘
 ```