[DOC] Updated documentations while preparing for DET calibration training

Description

While preparing for the Calibration training for the DET group I have updated the documentation to mention some troubleshooting steps in the training.

• Add troubleshooting pages.
• Add to CALCAT and myMDC pages.
• Add to webservice page.

How Has This Been Tested?

Relevant Documents (optional)

The updated documentation link can be accessed at: https://rtd.xfel.eu/docs/european-xfel-offline-calibration/en/doc-update_documentation/

The updated pages:

• CALCAT
• myMDC
• The webservice
• Added new troubleshooting pages: General and Detector specific

Types of changes

• Docs (changes to the documentation)

Checklist:

Reviewers

@kluyvert @schmidtp

docs/operation/calibration_database.md

@@ -103,7 +103,8 @@ constants files)
 
 PDUs can be transferred between detectors and instruments or replaced. To correct raw data, the calibration pipeline produces calibration constants, which are specific to the module they were generated from. As a result, it's crucial to map the hardware to the corresponding software modules.
 
-As the detector is 
+It's possible to update a detector mapping through CalCat in case the PDUs are already available. i.e. moving a PDU from one detector to another. Otherwise, only users with admin access can add entirely new PDUs.
+![detector_pdu_mapping_calcat](../static/calcat/detector_mapping_edit.png)
 
 
 ### Modifying detector mapping