docs: OEP-68 Learning Content Identifiers

[kdmccormick]

WIP

Preview: https://github.com/openedx/openedx-proposals/blob/kdmccormick/keys/oeps/best-practices/oep-0068-bp-resource-identifiers.rst

Abstract
********

When making references to learning content it's important to choose the right kind of
identifier and name it in a way that is obvious to others. We describe five
categories of identifiers: Intger Primary Keys, Codes, Opaque Keys, and UUIDs. We describe
when to use them and how to name them. Following these guidelines will help keep
the platform content coherent, correct, portable, and efficient.

The primary audience of this OEP is **developers working on openedx-core, openedx-platform
and other repositories which manage learning content**. However, plugin
developers, site administrators, and nontechnical Open edX Core Contributors
who interface with learning content may all benefit from understanding these
concepts.

[kdmccormick]

@ormsbee , could I ask you to give a very high-level review of what this OEP's trying to achieve? Don't worry about the exact prose and examples, I'd like to work on those a bit to make them more reader-friendly. Just want to make sure the scope and intent feels appropriate before I dig in to making a final draft.

[ormsbee]

@kdmccormick: I think the high level scope and framing are good. I was a little surprised to see UUIDs in there, but it makes sense. As part of this PR, you might want to amend OEP-38 so that people see a mention of this new OEP if they're looking at Data Modeling more generally.