Add long explanation, with install instructions

What does this PR change?

It was sometimes unclear that Kubecost has to be installed in the target cluster for kubectl-cost to function. This commit adds two things to kubectl cost help:

clarification of the need for Kubecost to be installed
a pre-formatted Helm install for Kubecost!

Does this PR rely on any other PRs?

N/A

How does this PR impact users? (This is the kind of thing that goes in release notes!)

Improved help information (with Kubecost install instructions) when running kubectl cost help

Links to Issues or ZD tickets this PR addresses or fixes

How was this PR tested?

Locally by running kubectl cost help and checking the output!

Have you made an update to documentation?

N/A

⚠️ GitGuardian has uncovered 1 secret following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secret in your pull request

| GitGuardian id | Secret | Commit | Filename | | | -------------- | ------------------------- | ---------------- | --------------- | -------------------- | | [3359586](https://dashboard.gitguardian.com/incidents/3359586?occurrence=35673363) | Generic High Entropy Secret | a183c1c42ff6fe6bf09258951129edc69f002441 | pkg/cmd/cost.go | [View secret](https://github.com/kubecost/kubectl-cost/commit/a183c1c42ff6fe6bf09258951129edc69f002441#diff-d20a80ae6635f0743cf4e8b5441852835bb1333179c374b6a590af9df015233cR125) |

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kubecost / kubectl-cost