WEB: Replace os.path with pathlib.Path in pandas_web.py
Replaces `os.path` with `pathlib.Path` in `pandas_web.py`, as suggested by @datapythonista in [61578](https://github.com/pandas-dev/pandas/pull/61578). No functional changes, verified site generation remains correct

- [x] Ran pre-commit check
Thanks for the review! updated to make sure all paths are created as `Path` early and used consistently. Removed redundant casts too. Addressed all the latest comments, switched to `Path.rglob`, cleaned up `base_url`, and ensured consistent `Path` usage. Let me know if anything else looks off, I also retested on local after the changes /preview Website preview of this PR available at: https://pandas.pydata.org/preview/pandas-dev/pandas/61604/ Thanks for the clean up here @iabhi4 , very nice PR