Add ADR to define default paths used by OS packages and agent

[soenkeliebau]

I've briefly documented what I believe should be the correct OS paths for some of the things we need to keep on disk.

I think this is correct according to https://refspecs.linuxfoundation.org/FHS_3.0/fhs-3.0.html but have my doubts, whether it makes sense in all cases.

Specifically I wonder about

1. /etc/opt/stackable - I have not often seen /etc/opt used for configuration.
2. Packages - do we want to keep these in /opt under our folder root as well?
3. Service configs, similar question, should those be somewhere else then /etc/opt/stackable/serviceconfig maybe a level up at /etc/opt/serviceconfig to clearly separate them from config for our components?

[soenkeliebau]

I've updated this and it is now ready for review @lfrancke

[lfrancke]

"Following the logic from above" two times in a row but with two different conclusions. I know what you're getting at but it reads weird.

Maybe something like "As outlined above" oder "According to the standard"

[soenkeliebau]

I have addressed a few comments, some are still left open due to a fundamental question about what we want to use ADRs for .
I was planning to update this ADR as we go along and have it be the source of truth for all os paths that the Stackable Platform uses, hence also included specifics in it.
@larsfrancke is of the opinion that we should keep this generic and leave implementation details out of it.

At the end of the day I am fine either way, but if we make this generic, then I think we should have a page detailing the paths somewhere else in the docs..

Anyone else have any thoughts on this?