[request] Document Artifactory Remote Repository

The Artifactory documentation is clear about [Remote Repository](https://www.jfrog.com/confluence/display/JFROG/Repository+Management#Remote+Repositories) functionality, where people can add `conancenter` as remote and download from there when a package is not available in local remote.

However, this information is not present to Conan users when they read Conan docs, and more than once, I saw people asking how to "copy" packages from Conan Center to local remote.

Of course, uploading manually all packages from local cache looks oblivious when you don't know about Artifactory features. We need a section explaining about it and pointing to Artifactory documentation.