Keep a good description for each tool in homebrew formular

**Describe the problem/challenge you have**
The current description of the tools is just the name as it is depicted in https://github.com/vmware-tanzu/homebrew-carvel/blob/develop/imgpkg.rb#L2
We should ensure that we have a good description for each tool even if we autogenerate the formula.

**Describe the solution you'd like**
When we generate the description based on the name of the tool in https://github.com/vmware-tanzu/carvel-release-scripts/blob/main/scripts/homebrew/homebrew_formula.yml#L44 maybe we should just keep a description in the releases.yaml file instead and preserve it.

**Anything else you would like to add:**
When we commit a change to releases.yaml file this action https://github.com/vmware-tanzu/carvel-release-scripts/blob/main/.github/workflows/update-install-sh.yml will be triggered, we should make sure that it doesn't have any unintended impact on the website.

---
Vote on this request

This is an invitation to the community to vote on issues, to help us prioritize our backlog. Use the "smiley face" up to the right of this comment to vote.

👍 "I would like to see this addressed as soon as possible"
👎 "There are other more important things to focus on right now"

We are also happy to receive and review Pull Requests if you want to help work on this issue.