[Docs] Improve k8s multi-cluster docs with exec-based auth information

[kyuds]

With #4379, it is much easier for users to use multiple k8s clusters. We should let them know in the docs.

Tested (run the relevant ones):

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)
• Relevant individual tests: /smoke-test -k test_name (CI) or pytest tests/test_smoke.py::test_name (local)
• Backward compatibility: /quicktest-core (CI) or pytest tests/smoke_tests/test_backward_compat.py (local)

[kyuds]

@romilbhardwaj I keep getting "unexpected indent on line 39 and 45" errors, just wondering what hte problem might be... Had a few debugging attempts but nothing seems to work