Add proper documentation instead of a single README file

[schloram]

I think it's better to keep the README slim and to add additional information (like https://github.com/itplusx/flexible_pages#2-registering-custom-pages-types or https://github.com/itplusx/flexible_pages#2-registering-custom-pages-types) as Documentation.

We could either use the GitHub Wiki for that or even the TYPO3 recommended way (https://docs.typo3.org/m/typo3/reference-coreapi/master/en-us/ExtensionArchitecture/Documentation/Index.html)