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.

[braver]

Perhaps related, I just dropped this in Discord, where it’ll probably get lost:

I've started digging into documentation around Package Control... Since nobody uses ST without PC, or certainly everyone who would browse the community docs has PC installed, the target audience with the community docs overlaps completely. In other words, why would package authors look in one place for how to build packages, and elsewhere on how to distribute packages? Especially since the one plays into the other quite a bit, and in my "job" as package reviewer I'm constantly referencing and linking to both.

All that to say, is it a good idea to make all PC docs part of the community docs?

Context: https://github.com/packagecontrol/thecrawl/issues/24

[michaelblyons]

https://github.com/sublimetext-io/docs.sublimetext.io/pull/134

[FichteFoll]

We now have a section with several pages regarding Package Control, including general usage, but installation is not yet mentioned. Reopening until that has been added.