System Tests README extended

[muhme]

Summary of Changes

• Extended with software architecture pictures
  • added new folder tests/System/images for used architecture diagrams, see README.md there
• Described software architecture as overview and second time in detail
• Detailed and updated the existing documentation
• Added hints for testing and development
• Added troubleshooting

Testing Instructions

• Are the used SVG images, intermediate PDFs and original ODGs stored in the right place?
  • and included in exlude list?
• Read the entire document
  • Everything correct?
  • Links are working?
  • Correct English?
  • Working on different browsers, device sizes, dark and light color mode?
  • All important information existing?
  • Are important concepts well explained?
  • Consistent notation of names in the document?
  • Are there possibly any permissions to consider?

Actual result BEFORE applying this Pull Request

• tests/System/README.md is existing, but is not up-to-date and the software architecture is missing

Expected result AFTER applying this Pull Request

• Joomla system tests are up-to-date documented, the software architecture is outlined and understandable
• All in all, it is a nice document and invites people to join Joomla ATT 😄

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

Upmerge

Please upmerge to branches 5.0-dev, 5.1-dev and 6.0-dev. All the information applies equally.

[laoneo]

@brianteeman when you are ok with the text, can you approve the pr?

[brianteeman]

I dont have the access to approve a pr but this PR is good enough for me

[laoneo]

Thanks!