[Docs] Add porting guide

[samccann]

Prerequisites

• [X] This was not already reported in the past (duplicate check)
• [X] It does reproduce it with code from main branch (latest unreleased version)
• [X] I include a minimal example for reproducing the bug
• [X] The bug is not trivial, as for those a direct pull-request is preferred
• [X] Running pip check does not report any conflicts
• [X] I was able to reproduce the issue on a different machine
• [X] The issue is not specific to any driver other than 'default' one

Environment

none

What happened

Molecule documentation is missing a porting guide or some other set of information on what has changed between releases and what a user needs to do to adapt to those changes.

See https://ansible.readthedocs.io/projects/project-onboarding/en/latest/docs_toolkit.html for general guidelines on what makes for complete Ansible project documentation.

Reproducing example

No response

[samccann]

Should be added to the references section in mkdocs.yml