Keep a good description for each tool in homebrew formular

[joaopapereira]

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.

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