Runbook

Create a runbook for jsii testing. The existing docs (READMEs and CONTRIBUTINGs) are somewhat difficult to use on their own, and don't include things like Python deprecations and adding new versions.

Review the existing README and CONTRIBUTING documentation to understand the current jsii testing workflow and identify the gaps. Create a runbook that explains testing, including Python deprecations and how to add new versions; it is done when these tasks can be followed without relying on the scattered existing docs.

Written by the indexing model from the issue text.