Revisit discussion about the version we use in PR preview builds

When we build PR previews in repos that contain docs content, we usually (always?) use whatever is on `main` in this repo. Should we use `main` or should we use the latest docs-builder release? It might be misleading to show behavior in the PR preview that's not going to be the same once the PR is merged.

cc @elastic/docs-engineering @elastic/docs-tech-leads