docs: Update requirements & tests manual

[scyzoryck]

Hello!
My first PR here. I made some small improvements to the setup documentation.
However, I might need some help with Running the tests section - After running tests/script_jackrabbit.sh it fails on the step of missing composer dependencies.

Best, scyzoryck.

[dbu]

thanks a lot for the cleanup! seems we forgot about the documentation when we moved away from travis-ci.

[scyzoryck]

Thanks @dbu for suggestions! I rewrote a bit section about setting up package for tests based on the commands from Github actions. :) Do you have some time for another look, please?

[dbu]

i realized that the documentation will be easier to read when explaining by implementation each step, rather than each step for each implementation. i reformatted it in #842

thanks to your work, this was quite easy to do now as you collected the necessary information and steps. thanks a lot!