Document the format of config & credentails files

[awsluja]

In addition to sharing files between the AWS CLI and other AWS SDKs, there are formatting rules that must be followed when configuring the config/credentials files, which must be documented.

Issue #, if available: This also addresses this issue, where the formatting of the config/credentials file came into question: https://github.com/aws-amplify/amplify-cli/issues/10525 .

Description of changes: Documented the formatting of the config & credentials files.

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