Replace link to the Python HOW-TO with a link to the Tink documentation site.
Replace link to the Python HOW-TO with a link to the Tink documentation site.

The PyPI package page (https://pypi.org/project/tink/) uses this
README.md file as the project description. Since this link uses a
relative path, it's broken when rendered on that site.

PiperOrigin-RevId: 460344851

google/tink,google/tink,google/tink,google/tink,google/tink,google/tink,google/tink,google/tink