Emphasize postgresql installation in User Guide

[andocz]

Following the instructions in the docs to set up PostgreSQL on Lubuntu 20.04, I got an error when running psql, saying a server wasn't running. It was not until I installed the postgresql package that the instructions worked. This package isn't installed by default on Ubuntu 20.04 or 18.04 desktop either.

I also changed leftover occurrences of apt-get to apt while I was at it.

[mcserep]

The deps.md contains the guide on how to compile CodeCompass. Installing a functional PostgreSQL server is not required for that, only the development package. Instead, the usage.md, the User Guide describes that a PostgreSQL server should be installed before usage.

Therefore I think the postgresql package was omitted deliberately from deps.md. What do you think @intjftw , @whisperity , @bruntib ?

[andocz]

Ah, my bad, I somehow missed that usage.md already told you to install postgresql-<version>, even though I edited the command for it...
But in my defense, all other command snippets in the documentation are inside code blocks, I think that's why I skimmed over it. Maybe we could change it to be a code block too to save a bit of confusion?

And I'm okay with keeping it out of deps.md. Someone writing CI scripts might use deps.md as reference, for example, and in that case installing it is unnecessary.

[whisperity]

Yes if the main package that also runs a server isn't needed, we should skip it. However, some sort of postgres is needed to build and generate the ODB bindings, no? Most likely that is only a dev package.

If the user manual for running wasn't clear, I'd suggest improving the visuals in that.

[bruntib]

Yes, I also think that deps.md is describing build dependencies only.
Using apt and apt-get in a uniform manner is a good point.

[mcserep]

Renamed the PR to reflect its updated content.