Move Architecture.md from Github wiki to documentation git repository

[lotharbach]

What would you like to be added:
concepts/architecture is currently sourced from Github wiki https://github.com/gardener/documentation/wiki/Architecture.md but would be better maintained directly inside gardener/documentation.git (or even gardener/gardener.git?)

Why is this needed:
The Wiki is not open to outside collaborators and does not support workflows like Pull requests. The License of the wiki content is unclear to me. This is the only documentation page that is maintained in a Github wiki.