Document placeholder of prerendering config

[ridoo]

Has to be documented at a better place:

https://github.com/52North/series-rest-api/pull/300

[ridoo]

for v2.x documentaion, first config note added in release notes wiki

[ridoo]

update on documentation: we decided to integrate documentation via "Jekyll, Kramdown, Swagger" Stack .. so it will be part of the sources

The documentation of prerendering config will be part of the configuration part and being closed by #383

[github-actions[bot]]

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