enforce wheel size limits, README formatting in CI

[jameslamb]

Description

Contributes to rapidsai/build-planning#110

Proposes adding 2 types of validation on wheels in CI, to ensure we continue to produce wheels that are suitable for PyPI.

• checks on wheel size (compressed),
  • to be sure they're under PyPI limits
  • and to prompt discussion on PRs that significantly increase wheel sizes
• checks on README formatting
  • to ensure they'll render properly as the PyPI project homepages
  • e.g. like how https://github.com/scikit-learn/scikit-learn/blob/main/README.rst becomes https://pypi.org/project/scikit-learn/

[jacobtomlinson]

LGTM