Improve documentation for setup of AGE extension

While trying to setup the extension, I did face a multitude of problems mostly due to the fact that I already had pg14 installed on my system and had to try to figure out on my own how to install pg11, change the pg_config file, and make sure I connect to the pg11 database using psql.

I hadn't found any proper resources on the internet on how to use multiple versions postgres on the same system without containers.

I would like to add to the documentation the steps to properly setup the age extension when having multiple versions of postgres, and request any regular contributors to guide me on how I could add to the documentation.