SNOW-1635365: Update snowpark pandas imports in method documentation

[sfc-gh-joshi]

1. Which Jira issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes SNOW-1635365 (resolves SNOW-1635365: ImportError when importing modin.pandas #2139)

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
     • If this test skips Local Testing mode, I'm requesting review from @snowflakedb/local-testing
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am adding new credentials
   • I am adding a new dependency
   • If this is a new feature/behavior, I'm adding the Local Testing parity changes.

3. Please describe how your code solves the related issue.

A few methods in documentation had an outdated import method for Snowpark pandas (import snowflake.snowpark.modin.pandas as pd). This PR updates docstrings to use the correct method (import modin.pandas as pd; import snowflake.snowpark.modin.plugin).

[sfc-gh-joshi]

FYI @sfc-gh-azhan @sfc-gh-nkrishna I had to make an additional change to the default argument of the typ flag of pd.read_json to get doctests to pass. CI was failing on src/snowflake/snowpark/modin/plugin/extensions/pd_overrides.py::snowpark.modin.plugin.extensions.pd_overrides.read_json, but the failure occurred only if the full doctest suite was run (not sure if there's a smaller subset that can trigger it). I suspect there might be some hidden conflicts between the read_* methods we define in snowflake/snowpark/modin/pandas/io.py and those we define in snowflake/snowpark/modin/plugin/extensions/pd_overrides.py; I'll investigate further in a separate PR if I have time before GA.