Write an OEP on changelog management

We currently aren't very consistent about managing changelogs in various Open edX repositories, and this lack of consistency (or even basic availability) often makes it hard for us and other Open edX community members to stay informed about important changes to the code we're working on or considering upgrading. Write an OEP with some recommendations to improve this, including:

• Tooling for preparing changelog entries in PRs with minimal risk of merge conflicts (towncrier or equivalent)

• Preferred changelog format and structure (see https://keepachangelog.com/en/1.0.0/ as a starting point)

• Identification of changelog URL in package metadata (for PyPI and preferably NPM also)

• Guidelines on what to include (and exclude) in changelogs