Add history section to externally managed environments spec. - #1308

@chrysle it seems like that breezy website needs to be added to linkcheck_ignore: https://lists.ubuntu.com/archives/bazaar/2023q4/076422.html.

Also, looking into the merge queues behavior: it looks like GH generates 2 events for it — one is merge_group and the other is push. This triggers 2 CI runs — https://github.com/pypa/packaging.python.org/actions/runs/6614309433 and https://github.com/pypa/packaging.python.org/actions/runs/6614309436 respectively.
We also have concurrency groups set up in a way that results into both events sharing the same group identifier. So the push CI workflow cancels the merge_group one. We may want to adjust this too...

This https://github.blog/changelog/2022-08-18-merge-group-webhook-event-and-github-actions-workflow-trigger/ says

A push event is still sent when a merge group branch is created, and will trigger a GitHub Actions workflow. However, unlike a merge_group event, a push event does not include the target branch of the merge group.

Which kinda implies that we may need to drop the push event.

So the push CI workflow cancels the merge_group one.

Ah, and that cancellation prompts GH to consider the CI run a failure, leading to destroying of the merge queue and unqueueing the PR.

@chrysle #1318 + #1319 should improve things.