docs: pgbouncer for postgresql replica

[davidumea]

⚠️ IMPORTANT ⚠️: This is a public repository. Make sure to not disclose:

• personal data beyond what is necessary for interacting with this Pull Request;
• business confidential information, such as customer names.

Quality gates:

• I'm aware of the Contributor Guide and did my best to follow the guidelines.
• I'm aware of the Glossary and did my best to use those terms.

Important

Links to code snippets reference line numbers.
If you changed the NodeJS user demo or the DotNET user demo, then please search for all links to code snippets and update them accordingly.
You can find such links with egrep -R '\[.*\]\(.*user-demo.*#L.*)' docs.

• I have updated links to code snippets or I haven't changed code snippets.

Part of https://github.com/elastisys/compliantkubernetes-postgresql/issues/306

[elastisys-staffan]

👍

[andremarklund-elastisys]

LGTM! 👍