Proposed changes

I've received feedback that it should be more obvious that Users need to Edit Widgets / Adapt the UI before seeing the custom fragments they created via thin-edge.io.
For all other actions (create measurements, events, alarms, create child-devices/services, etc.) no changes are needed on cloud side, for custom fragments there is. This PR introduces a short note about it to the docs.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s. You can activate automatic signing by running just prepare-dev once)
• I ran just format as mentioned in CODING_GUIDELINES
• I used just check as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments