installing/cloud: add note against installation using AWS VMDK image

[dongsupark]

Add note about not recommending installation using AWS VMDK image, due to intermittent AWS AMI corruption issues.

[tormath1]

Hello, this repo is now archived. This PR should be re-opened against: https://github.com/flatcar/flatcar-website.

To update your commit(s):

PR_ID=334
cd flatcar/flatcar-website
wget "https://patch-diff.githubusercontent.com/raw/flatcar/flatcar-docs/pull/${PR_ID}.patch"
sed -i 's|/docs/|/content/docs/latest/|g' "${PR_ID}.patch"
git am "${PR_ID}.patch"