[IMP] install: add $connection_upgrade map documentation

[tsm-odoo]

Before this commit, following the install documentation to setup the odoochat server would fail. This was due to the fact that the documentation about how to define the $connection_upgrade variable was missing.

This PR fixes this issue.

[robodoo]

Pull request status dashboard.

[tsm-odoo]

Hello @AntoineVDV, could I please have your review on this ? Thanks :+1:

[Feyensv]

@robodoo r+