Decide & document the smithy-swift versioning policy

[jbelkins]

Describe the issue

Describe the issue

The smithy-swift versioning policy should be:

• Agreed upon by the team, and
• Documented in the project so that customers can understand & manage change in future smithy-swift releases.

Links

https://github.com/awslabs/aws-sdk-swift/blob/main/Sources/Core/AWSSDKForSwift/Documentation.docc/stability.md (Existing SDK policy)
https://docs.aws.amazon.com/sdkref/latest/guide/maint-policy.html (SDK & Tools org policy)

[sichanyoo]

The ticket consists of driving discussion for the consensus, making any code change as needed, and documenting the agreed upon approach.

[jbelkins]

Removing from current sprint; smithy-swift versioning will remain unchanged for now.