Add pyparsing porting guide

[1cbyc]

Summary

• add a new guide for porting pyparsing parsers to Lark
• cover common pyparsing-to-Lark equivalents, parse actions to transformers, ignored text, delimited lists, precedence, and Lark.scan()
• add the guide to the Tutorials and Guides toctree

Closes #1008

Verification

• Ran the Python snippets for the minimal parser and Lark.scan() examples locally.
• Built docs with:
  uv run --with-requirements docs/requirements.txt sphinx-build -b html docs docs/_build/html

Notes: full docs build succeeds with existing unrelated warnings in generated example/composition pages and other current docs. The new pyparsing_porting page produced no warnings after adjustment.