Update Citus Downgrade documentation

[jfensterman]

Why are we implementing it?

Improve quality of life for users that need to downgrade citus versions

What are the typical use cases?

For any user of Citus that needs to downgrade major/minor versions due to a failure or mistaken upgrade

Communication goals (e.g. detailed howto vs orientation)

Detailed how to with examples (see attached)

Downgrade Process.docx