docs: Refactor README and add experimental feature warning

This commit significantly improves the project's documentation and user interface to provide a clearer and safer experience for new users.

- README.md:
  - Restructured the entire document to follow a more logical flow for first-time users: Installation -> Configuration -> Usage.
  - Added an explicit 'Prerequisites' section (Python, Git, Chrome).
  - Strengthened the language around critical Notion setup and database field configuration to prevent common errors.
  - Added a warning for the experimental nature of batch processing scripts.
  - Overall, the README is now more comprehensive and user-friendly.

- gui/widgets.py:
  - Added a prominent, styled warning label at the top of the 'Batch Tools' tab.
  - This visually alerts GUI users that these functions are experimental and advises them to back up their data, aligning the GUI with the updated README.

Author: baiheyyds