Add Kaggle Secrets documentation (#1131)

Expose Kaggle Secrets usage in the CLI documentation to help users avoid
hardcoding sensitive credentials in their notebooks.

Added a new section "Using Secrets in Kernels" to docs/kernels.md and
linked to it from docs/README.md.

TAG=agy
CONV=2a820637-152b-46a6-9a45-e29918f1e322

Fixes #https://github.com/Kaggle/kaggle-cli/issues/408