Add a page for Package Control (and other methods for installing third-party packages)

[FichteFoll]

I would like to have an internal page that we can reference for all things Package Control, which is useful for the packages guide page but also relevant whenever a package should be installed, such as PackageDev. It should also cover the current PC3 vs PC4 situation and include manual update steps (copyable from #faq on the Discord) because the pc.io site cannot be updated at the moment.