adds a new release notes subnav and removes old api docs
adds a new release notes subnav and removes old api docs

cloudfoundry/docs-book-cloudfoundry,cloudfoundry/docs-book-cloudfoundry,swisscom/docs-appcloud-book,cloudfoundry/docs-book-cloudfoundry,swisscom/docs-appcloud-book,swisscom/docs-appcloud-book,cloudfoundry/docs-book-cloudfoundry,swisscom/docs-appcloud-book