Implement the new ReleaseProcess.md plan

[nfrisby]

• add a document explaining our two package bundles and why
• initialize an empty CHANGELOG.md, one per bundle
• add a Consensus CHANGELOG.md file that merely directs the reader to the two CHANGELOG.md files, one per bundle
• each per-bundle CHANGELOG.md should start with a warning: "if you don't see here the package you're interested in, see the top-level Consensus CHANGELOD.md"
• add a requirement to the PR description template that says you must use scriv to add an entry to the CHANGELOG.md for any bundle that includes a package that your PR touches; the entry must at the very least classify the changes as exactly one of Patch-level, Non-Breaking, or Breaking. "See ReleaseProcess.md for more details."
• Would scriv's "Categories" be useful for the required classifications? https://scriv.readthedocs.io/en/1.1.0/concepts.html#categories