PSA: use `make watch-secure` for UI dev against secure clusters

[Pete Vilter]

(If you never use `make watch` in `pkg/ui`, this doesn't affect you)

The `make watch` rule still works the same.

However, I have introduced `make watch-secure`, which instructs the webpack proxy to use HTTPS and hit an HTTPS URL on your cockroach cluster. This fixed strange behavior I was seeing when using `make watch` against a secure cluster (invalid responses, redirect loops, etc)

This is documented in pkg/ui/README, but just posting here as well.