`mdbook serve` clobbers old versions of docs

[alexdewar]

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.

[alexdewar]

Looks like it's not limited to the release folder. Turns out mdbook will clobber other folders too: #1366 (comment)

So the "fix" for this may be to just not run mdbook serve when working with the "old docs"...

This shouldn't affect most developers though -- generally you only want to modify the latest version of the docs, which works fine with mdbook serve. It's only a problem if you're playing with the scripts.