Reuse Ubuntu PostgreSQL installation template for the RPi 3 installation guide

[xalt7x]

PR description

Instruction from https://thingsboard.io/docs/user-guide/install/rpi/ currently suggests to install "postgresql" meta-package. This package depends on recommended version from the PostgreSQL repository (currently, "postgresql-16").

Other similar Thingsboard installation guides recommend "postgresql-15". "postgres-install-ubuntu.md" has correct information, fits for Ubuntu, Debian and Raspbian distributions, and except that, doesn't have any differences from "postgres-install-rpi.md". So let's just reuse "postgres-install-ubuntu.md" for Raspberry Pi 3 page

Link checker

The links will be checked by the build agent automatically once you create or update your PR.

You can use the following command to check the broken links locally.

docker run --rm -it --network=host --name=linkchecker ghcr.io/linkchecker/linkchecker --check-extern --no-warnings http://0.0.0.0:4000/

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Yevhen Popok seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}