docs: first tutorial on quickstart CNPG

[jbattiato]

This is the first of many blog posts about tutorials.
The purpose is to extract useful hands-on guides from the documentation, which is becoming a bit chaotic.

[jsilvela]

Great! Thanks Jonny.
First of all, I see this is wanting to merge into production ... which is generally not what we do. We usually merge to main, then Gabriele cherry picks into production.

And, I think we should also get the user to a psql prompt, some basic commands to show them it's the PG they know and love.
Perhaps if we dare, show quickly adding a replica or forcing a failover.

[jsilvela]

@sxd you asked for a new review about this? Left comments a year ago that I still stand by.