add document on how to migrate from v3

[davidbrota]

Description of the changes

Add a document to describe the changes and how to adapt an existing integration to the new SDK.

PR Review Checklist

Author

• [ ] add a risk label after carefully considering the "blast radius" of your changes
• [ ] describe the intent of your changes in the description. don't just rewrite your code in prose
• [ ] assign at least one reviewer
• [ ] document feature

Reviewer

• [ ] review code for readability
• [ ] verify that high risk behavior changes are well tested
• [ ] check license for any new external dependency
• [ ] request documenation about anything that isn't clear and obvious
• [ ] document agent version supporting the feature

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

David Brota seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}