TUI: fix MCP docs hyperlink in empty_mcp_output (#2907)

- Summary:
- Updated the hardcoded hyperlink shown when no MCP servers are
configured to point at the canonical docs section:
    - From: codex-rs/config.md#mcp_servers (moved/obsolete)
    - To: docs/config.md#mcp_servers (correct GitHub path)
- Rationale:
- The TUI link was pointing to a file that only redirects; this makes
the link accurate and reduces user confusion.
- Validation:
- Verified that the target anchor exists at:
https://github.com/openai/codex/blob/main/docs/config.md#mcp_servers
- UI behavior unchanged otherwise (rendering of link text remains “MCP
docs”).
- Impact:
- One-line change in TUI display logic; no functional behavior change.

Co-authored-by: Michael Bolin <mbolin@openai.com>

codex-rs/tui/src/history_cell.rs

@@ -1032,7 +1032,7 @@ pub(crate) fn empty_mcp_output() -> PlainHistoryCell {
         "  • No MCP servers configured.".italic().into(),
         Line::from(vec![
             "    See the ".into(),
-            "\u{1b}]8;;https://github.com/openai/codex/blob/main/codex-rs/config.md#mcp_servers\u{7}MCP docs\u{1b}]8;;\u{7}".underlined(),
+            "\u{1b}]8;;https://github.com/openai/codex/blob/main/docs/config.md#mcp_servers\u{7}MCP docs\u{1b}]8;;\u{7}".underlined(),
             " to configure them.".into(),
         ])
         .style(Style::default().add_modifier(Modifier::DIM)),