feat: add cross-account runbook + actions

[svia3]

Issue #, if available:

Description of changes:
Add x-account instructions + runbook for walkthrough of setup + script actions.

Testing done:
Manually using hulk test accounts

Merge Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your pull request.

• I have verified that my PR does not contain any new notebook/s which demonstrate a SageMaker functionality already showcased by another existing notebook in the repository
• I have read the CONTRIBUTING doc and adhered to the guidelines regarding folder placement, notebook naming convention and example notebook best practices
• I have updated the necessary documentation, including the README of the appropriate folder as well as the index.rst file
• I have tested my notebook(s) and ensured it runs end-to-end
• I have linted my notebook(s) and code using python3 -m black -l 100 {path}/{notebook-name}.ipynb

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.