Publish-IcingaPluginDocumentation is clearing entire docs on creation

[LordHepipud]

By using Publish-IcingaPluginDocumentation we automatically generate the documentation for each plugin for GitHub/Icinga.com.

How ever, right now we are flushing the entire folder content and re-creating each single file with a new index, making it incompatible with possible old links to docs. For example if we are having the plugin doc file 01-Invoke-IcingaCheckBiosSerial.md and create a new plugin like Invoke-IcingaCheckAsSomething this one would be moved to index 1, while the Bios plugin be moved to Index 2.

This makes it hard to keep track on docs and links as the entire doc structure is regulary changing. We should check if a plugin doc file already exist and update that one and only create new files for new check commands with a new index if neccessary.