Document the PG upgrade process in the Database docs

Leonardo da Rosarequested to merge
524428-pg-upgrade-timeline into master

What does this MR do and why?

Document the PG upgrade process in the Database docs.

Adds a new section with the cross-team efforts to upgrade the PG version

Example docs: https://docs.gitlab.com/upstream-review-mr-ee-188480/development/database/pg_upgrade_timeline/

Related to #524428 (closed)

Edited by Leonardo da Rosa

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