Docs improvements

[sznowicki]

As signalled over Matrix I prepared some docs changes that I believe would make me less confused during my initial setups where I wanted to have two agents: docker and local and pick the proper one per pipeline jobs.

On top added some references here and there in places where I felt confused as well, while not being that experienced with linux installation process and systemd setups.

Thanks again for help yesterday!

[xoxys]

Thanks! After looking into this, I have the feeling we should refactor the installation guide at all. The added installation instructions are not specific for the local backend and should be part of the installation instructions for package-based installations. However, this docs part is pretty messy, there is only a section for third party packages while DEB and RPM packages are officially supported...

[qwerty287]

I agree to @xoxys - thanks for the contribution, but this is not the right place and the whole part needs rewriting. The issue with the direct binary installation is also tracked in #1292

Besides that, it is not necessary to separately build the plugins. You can just run pnpm build in the docs repo and this builds the plugins as well.

[xoxys]

Ill try to schedule some time this week to refactor the admin docs.