fix: remove HTML exports from docs directory

- Remove dataframe_example.html, itemset_example.html, sequence_example.html
- Update examples.md to link to notebooks on GitHub instead of HTML files
- Update notebooks/README.md to remove HTML export instructions for docs

Addresses feedback that HTML files in docs/ directory are not needed.

Co-authored-by: jacksonpradolima <7774063+jacksonpradolima@users.noreply.github.com>

Author: Copilot