Ability to include stack navigation links in PR description

We should provide the ability for users to opt into including the stack navigation links in their PR description instead/in addition to a stack navigation comment.
It's often useful to retain that the commit was part of a stack once merged. (Folks squash-merging will use the PR description more often than not.)

UX-wise, this probably takes the form of a `` in the PR description.
If found on its own line, we will generate or update a stack navigation visualization under it.
We'll want to be careful about the "update" part to only match if the thing following the HTML comment is a Markdown list.

This should probably be opt-in because to avoid the additional extra API calls.

Depends on #331

Das Issue hängt von #331 ab; beginne daher damit, diese Voraussetzung zu verstehen. Suche nach Code, der Kommentare zur Stack-Navigation und PR-Beschreibungen erzeugt. Das Feature umfasst das Parsen von PR-Beschreibungen nach einem -Marker und das Aktualisieren von Markdown-Listen. Prüfe, wie das Tool API-Aufrufe durchführt, um unnötige Aufrufe zu vermeiden, wenn dies opt-in ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.