Migrate general Fluid information to TYPO3 Explained

[sypets]

Blanket issue for several PR which will migrate most of the general Fluid chapter (which is not specific to Extbase) to TYPO3 Explained Fluid.

Since Fluid now plays a more general role and is also used for example for templating in combination with FLUIDTEMPLATE, in emails etc., Fluid is no longer mostly used in combination with Extbase - thus it does not make as much sense anymore, to keep the general Fluid parts in this book.

Additionally, we discussed the idea of creating a new (more compact) Extbase manual within the extbase core system extension, since the book has a number of shorcomings, most of which is duplication of content.

topic	old	new	status	redirects
inline syntax: moved to syntax page	old	new	done: merged PR removed: PR	done
import viewhelper namespaces: moved to syntax page	old	new	done	-
Developing a custom ViewHelper	old	new	done: merged: PR, removed	done
cObject ViewHelper	old	new	done: merged: PR, remove: PR	done
Fluid comments => syntax	old	new	done: migrated: PR, removed	no redirect necessary
Boolean conditions	old	new	migrated: PR, removed	done

• import ViewHelper namespace was within section "Developing a custom ViewHelper" but is now moved to Fluid syntax
• Most sections have been shorted as much as possible on migration