Update tests docs

[MakisH]

Some more information:

• How to trigger the test manually from the GitHub CLI
• General architecture (first draft)
• What each relevant file is doing

Also removes the mention about _TAG naming rule, as I understand it is not used anywhere.

@valentin-seitz I would still like to add (but I am not yet sure how):

• How to specify which git reference of each component to test?
• How to find out which versions (git references) were used for the test? -> needs Report git hash of each repository #401
• Which files to read when things go wrong?
• How to increase the verbosity of the tests? How to increase the verbosity of Docker?
• How to generate reference results for specific tests? -> needs Trigger generation of systemtests reference _results #405

Maybe some of these need to go to separate issues.

[MakisH]

@valentin-seitz Merging for demonstration purposes, feel free to update later.