Allow projects to modify `modified` workflow templates with a YML

[ArBridgeman]

Summary

In #639, templates were marked as either modified or generated. We now want to allow modified templates to be sanely modified by either having REPLACE or INSERT_AFTER actions like the project-keeper:

https://github.com/exasol/project-keeper/blob/main/doc/user_guide/user_guide.md#customize-workflow-environment

To Do

• Create a defined YML format similar to that used in project-keeper:
  https://github.com/exasol/project-keeper/blob/main/doc/user_guide/user_guide.md#customize-workflow-environment
• Define the filepath of this YML file as a default in the BaseConfig
• Update the nox session workflows:update so that uses a function(s):
  • to load the YML & validate it has the correct structure
  • it verifies that the specified workflow template to be modified is modified. If it's not (either custom or generated), raise an exception.
  • goes through the specified workflow template & searches for the id. If it's not found, raise an exception
  • if the id is found, perform the action specified REPLACE or INSERT_AFTER
• Add tests to verify that all the parts work as expected and that the end result of a modified YML behaves as desired.
• Like in Mark workflow templates as generated or modified #639, add the user & developer guides, using project-keeper as inspiration, about how modified templates are supposed to work.