[Change] Warning not to use TimescaleDB builds from Postgres repo (apache version)

[mfreed]

Users are told to go to the Postgres repo's to get postgres, but they might also install TimescaleDB from there. But those versions are Apache only and not built by Timescale, might be useful to have some warning in the docs.

[gusennan]

I'm running Fedora 33 and tried to follow the instructions for installing RPMs from this page:

https://docs.timescale.com/latest/getting-started/installation/rhel-centos/installation-yum

After adding TimescaleDB's package cloud repo, I did not see the packages mentioned in the docs, but I did see the ones from the postgres repo and almost installed from there.