fix loading of custom ruby extensions

[PacoVK]

This will fix #1290 #1239 and also complete the performance issue described in #1353

This PR moves the setting rubyExtensions to the top level and makes it no longer an option within the microsites config. The reason is that it does not relate to micrositetasks since they depend on JBake and JBake currently does not support custom Ruby extensions.

A follow up to this task is #1363 where we should document the config option

All Submissions:

• [x] Did you update the changelog.adoc?
• [x] Does your PR affect the documentation?
• [x] If yes, did you update the documentation or create an issue for updating it?

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[PacoVK]

@rdmueller PTAL if you have time :)

[rdmueller]

Shouldn't we keep the mechanism that the gems are automatically activated when we use reveal?

[PacoVK]

@rdmueller thats what this PR does activate Ruby gems only when actually needed. Otherwise you see dramatic loss of performance in every docToolchain task due to the start of the required Ruby Gem proxy if defined globally...

[rdmueller]

let's merge...