Docs:Add bypassing PgBouncer link

What does this MR do and why?

The MR adds a link to the Update bypassing PgBouncer section of the Zero downtime upgrades documentation. This was necessitated by a customer who did not understand how to bypass PgBouncer as outlined in the Zero downtime upgrades section.

The section I have linked outlines the steps to bypass PgBouncer in clear and concise steps.

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MR acceptance checklist

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