fix: clone CLI chart to /tmp to avoid submodule and copy helm docs correctly

[dangrondahl]

Fixes two issues with the helm docs generation step:

• Using actions/checkout with path: cli-repo placed a nested git repo inside the docs workspace, causing create-pull-request to stage it as a submodule. Replaced with a plain git clone to /tmp/cli-repo which is outside the tracked workspace.
• helm-docs --output-file is relative to the chart directory and cannot write outside it. Instead, generate to the default README.md in the chart dir and copy it to helm/k8s_reporter.md in the docs repo.