`mdbook serve` clobbers old versions of docs

Not an especially important bug.

I often run `mdbook serve -o` to view the documentation once I've built it, but if you build the old docs (`just build_docs::all_with_old`) then run `mdbook`, the old docs are deleted again. The problem is that we have a folder at `docs/release` which just contains some Python code, but `release` is also the name of the folder where we put old versions of docs when we generate them. When you run `mdbook serve` it clobbers the contents of `book/release` with `docs/release`.

You can just open `book/index.html` directly instead. We could fix this by renaming `docs/release` to something else.

Start by inspecting docs/release and the build_docs::all_with_old recipe in just, then reproduce the conflict with mdbook serve -o. Identify references that depend on the docs/release path and choose a non-conflicting name. Done means building old documentation and running mdbook serve no longer removes book/release or its contents.

Written by the indexing model from the issue text.