Add section on stacking pull requests to Pull Requests guide

If you have a set of changes that you need to split up into separate stages, and some of those stages are dependent on each other, you'll need to create PRs that depend on each other. There's some specific advice around creating stacked pull requests. Specifically,

- keep all PRs in the stack except the first in draft (because of the next point)
- you never want to merge a PR that's not the first in the stack into another that's also not the first — when the first PR is merged, you need to rebase all other PRs

Open the Pull Requests guide and locate the section covering dependent or stacked pull requests. Add the listed advice about keeping later PRs in draft, avoiding merges between non-first PRs, and rebasing the remaining stack after the first PR merges. Done means the guide clearly explains these practices.

Written by the indexing model from the issue text.