Tweak modularize script

1.) Ignore .json files. On mac, every depfile starts with
    SDKSettings.json, leading to this error:

    mac-x64 raised an exception:
    concurrent.futures.process._RemoteTraceback:
    """
    Traceback (most recent call last):
      File "/usr/lib/python3.13/concurrent/futures/process.py", line 254, in _process_worker
        r = call_item.fn(*call_item.args, **call_item.kwargs)
      File "/usr/local/google/home/thakis/src/chrome/src/build/modules/modularize/modularize.py", line 148, in _modularize
        graph = compiler.compile_all()
      File "/usr/local/google/home/thakis/src/chrome/src/build/modules/modularize/compiler.py", line 51, in new_fn
        result = fn(self, *args)
      File "/usr/local/google/home/thakis/src/chrome/src/build/modules/modularize/compiler.py", line 294, in compile_all
        kind, _ = self.split_path(abs_path)
                  ~~~~~~~~~~~~~~~^^^^^^^^^^
      File "/usr/local/google/home/thakis/src/chrome/src/build/modules/modularize/compiler.py", line 119, in split_path
        raise NotImplementedError(f'Unsupported path {path}')
    NotImplementedError: Unsupported path /usr/local/google/home/thakis/src/chrome/src/build/mac_files/xcode_binaries/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/SDKSettings.json
    """

2.) Use "python3" in run line

3.) Update documentation to be up-to-date after
    https://chromium-review.googlesource.com/c/chromium/src/+/6883372

Change-Id: Id2e36a1d621947408d6ac0e972d99e38f633a8ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7786175
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Matt Stark <msta@google.com>
Cr-Commit-Position: refs/heads/main@{#1619284}
NOKEYCHECK=True
GitOrigin-RevId: 8c5aef6e66e82ea1d9a4ab7e872b5bcde0ab233a