Add 9.0 Installation and Upgrade Guide

[lcawl]

This PR adds a 9.0 branch to the installation and upgrade guide, so that we have a place to temporarily publish the 9.0 release notes and serverless changelogs during the documentation migration.

[github-actions]

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[karenzone]

Failed, but looks like an easy fix. :-)

INFO:build_docs:Live branch(es) <9.0> not in <branches> in book <Installation and Upgrade Guide> at lib/ES/Book.pm line 127.
--
  | 🚨 Error: The command exited with status 25
  | user command error: exit status 25