Add Migration Guide for the octopusdeploy_project.versioning_strategy deprecation

[mjhilton]

In v0.37.1 we deprecated the octopusdeploy_project.versioning_strategy attribute in favour of a new dedicated resource, but customers have been getting tripped up on the migration process. This seems like a perfect opportunity to retrospectively implement our newly minted Breaking Changes policy, which didn't exist when this deprecation was performed.

We had a few customers running into an issue migrating from the old approach to the new, due to a bug in the schema. This prompted us to think about the customer experience for the migration, which would have been improved if we had have written a migration guide - it would have forced us to think a bit more deeply about the customer migration scenarios, and possibly catch this bug earlier.

The bug has been fixed, but I've gone through the process of writing a migration guide after-the-fact, which can serve as a template for our future breaking changes.

This PR:

• Adds a Breaking Changes list under "Upgrades & Migrations"
• Adds a Migration Guide for v0.37.1
• Updates the deprecation message for octopusdeploy_project.versioning_strategy to include a shortlink to the migration guide
• Revives a very old Migration Guide for v0.8.0, which was never actually published on the Registry documentation due to it being in an incorrect location in the folder structure. This is just for historical accuracy.