Add guidelines for maintaining changelogs

Currently we do not have enough documentation on changelogs: what format we follow, why they are useful, and how to keep them in good shape. We have found especially on the Wallet Framework side that as more people contribute to the `core` repo they often group too many entries together when they should split them up, they don't provide enough detail around API changes, they include developer-only changes, or they allow commit messages to get copied straight over (along with Conventional Commit prefixes) without rewording them.

We ought to document all of the guidelines we internally follow so that anyone who updates a changelog in any MetaMask project can make use of them.