Add doctest infrastructure and fix failing test

Phase 1 & 2 of doctest improvements:
- Fix import path in chipflow-toml-guide.rst doctest (_parse_config_file from config.parser)
- Add doctest_global_setup in conf.py with common imports for test examples
- Existing doctest now passes

The global setup imports common modules (Amaranth, ChipFlow signatures) so
that doctests can focus on demonstrating API usage without repetitive imports.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: robtaylor

docs/chipflow-toml-guide.rst

@@ -14,7 +14,7 @@ Let's start with a typical example:
    # Assert that example-chipflow.toml matches the current config schema. If
    # this test fails, then its likely that the content in this file will need
    # to be updated.
-   from chipflow_lib.config import _parse_config_file
+   from chipflow_lib.config.parser import _parse_config_file
    _parse_config_file("docs/example-chipflow.toml")
 
 ``[chipflow]`` table

docs/conf.py

@@ -108,6 +108,20 @@
     ("Members", "params_style"), # `amaranth.lib.wiring` signature members
 ]
 
+# Doctest configuration
+doctest_global_setup = """
+from pathlib import Path
+from amaranth import Module
+from amaranth.lib import wiring
+from amaranth.lib.wiring import In, Out, connect, flipped
+from amaranth_soc import csr, wishbone
+from chipflow_lib.platforms import (
+    UARTSignature, GPIOSignature, SPISignature, I2CSignature,
+    QSPIFlashSignature, JTAGSignature,
+    IOTripPoint, Sky130DriveMode,
+    SoftwareDriverSignature, attach_data, SoftwareBuild
+)
+"""
 
 rst_prolog = """
 .. role:: py(code)