Merge templates.xml and context.xml into one file

[MikeSchumacherCapgemini]

With the new requirement of splitting the configuration into multiple template-sets, there is no necessity of maintaining a context.xml and templates.xml anymore. Especially, as there is no understanding by the user why and in which way these two configurations are separated.

Acceptance Criteria

• [x] the new combined config file name should be template-set.xml
• [x] Automatic upgrade is adapted to merge the configurations resulting in template-set.xml
• [x] #1522
• [x] documentation of configuration is adapted accordingly

[maybeec]

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