Polish standalone docs and add notebook examples

[mhmdaskari]

Summary

• Reframes README and documentation around eegclassify as a standalone Python package.
• Removes legacy/course/notebook-derived wording from public docs and generated API docstrings.
• Keeps the course-origin report link and archived notebook pointer only at the end of the README.
• Adds rendered notebook examples under docs/examples/ and wires them into MkDocs navigation with mkdocs-jupyter execution disabled.
• Updates notebook narratives and reference-target labels to avoid legacy-baseline wording.

Validation

• rg -n "legacy|built from|built on|original notebooks|notebooks/legacy|course split|legacy parity|original notebook|original project|C247|Report\\.pdf" README.md docs mkdocs.yml notebooks src -g '!notebooks/legacy/**'
  • Only the two allowed README ending lines remain.
• CUDA_VISIBLE_DEVICES=-1 mkdocs build --strict
• git diff --check
• python3 -m compileall -q src
• git check-attr linguist-detectable -- docs/examples/00_data_postprocessing.ipynb notebooks/00_data_postprocessing.ipynb