Support specifying baseline in configuration file

[AryanBagade]

Summary

Adds support for setting the baseline file path in pyrefly.toml or pyproject.toml, so users don't need to pass --baseline on every invocation.

  # pyrefly.toml
  baseline = "baseline.json"

• Add baseline field to ConfigFile struct
• Read baseline from config when CLI flag not provided
• CLI --baseline takes precedence over config
• Update documentation

Fixes #2119

Test Plan

• Added unit test for parsing baseline from config
• Updated test_rewrite_with_path_to_config to include baseline path
• Ran python3 test.py -> all tests pass
• Manual testing: verified baseline works from config, CLI override works

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[meta-codesync]

@grievejia has imported this pull request. If you are a Meta employee, you can view this in D90895437.

[yangdanny97]

Review automatically exported from Phabricator review in Meta.

[meta-codesync]

@grievejia merged this pull request in ce020ef.