Improve scheduled publishing docs

[laymonage]

I forgot (sorry!) to document that it's required to include PublishingPanel() in the snippet model's panels definition in order to use the scheduled publishing feature introduced in #8976.

This PR also improves the docs on how the schedule publishing mechanism works in Wagtail. Most notable change is that we should explicitly state that clicking "Publish" is required for the schedule to actually be scheduled.

I also added some explanations regarding some edge cases for scheduled publishing when an unpublishing schedule has already been set. Not sure if it's worth explaining this one, but I think it's better than not knowing what happens.

[squash-labs]

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Test this branch here: https://laymonagesnippets-schedule-pub-safa0.squash.io

[gasman]

I'm sure I saw a line that added PublishingPanel to DraftStateMixin snippets automatically - did that get removed for the final version of #8976?

Incidentally, I did notice a related quirk when testing this on bakerydemo - I tried it on the Country model, which doesn't define a panels attribute, and so Wagtail supplies a default one consisting of an ObjectList with FieldPanels for all editable fields. On adding DraftStateMixin, this caused the go-live / expiry date fields to show up in the main body of the form. I did wonder about fixing that by setting those fields to editable=False, but figured that: 1) that might cause bigger side effects elsewhere; 2) this quirk will only surface when first adding DraftStateMixin to a model, and the fix is obvious; and 3) I'm not sure if omitting panels is even an officially supported setup...

[laymonage]

I'm sure I saw a line that added PublishingPanel to DraftStateMixin snippets automatically - did that get removed for the final version of #8976?

Incidentally, I did notice a related quirk when testing this on bakerydemo - I tried it on the Country model, which doesn't define a panels attribute, and so Wagtail supplies a default one consisting of an ObjectList with FieldPanels for all editable fields. On adding DraftStateMixin, this caused the go-live / expiry date fields to show up in the main body of the form. I did wonder about fixing that by setting those fields to editable=False, but figured that: 1) that might cause bigger side effects elsewhere; 2) this quirk will only surface when first adding DraftStateMixin to a model, and the fix is obvious;

Ah yes, I did remove the line that added PublishingPanel because I remembered having a discussion with Karl and Jacob and we thought that it'd be better to explicitly require the PublishingPanel() instead of it being magically added somewhere by Wagtail.

and 3) I'm not sure if omitting panels is even an officially supported setup...

I think the snippets docs always explicitly show the panels definition in the models. In addition, we added the following admonition to the docs in 4.0:

wagtail/docs/topics/snippets.md

Lines 358 to 362 in 01c2508

	```{note}
	The `DraftStateMixin` includes fields used by Wagtail's publishing mechanism that may currently be inapplicable for snippets. For example, the scheduled publishing fields (i.e. `go_live_at`, `expire_at`, and `expired`) are added to snippet models that inherit from the mixin, but the scheduled publishing feature itself is not yet officially supported for snippets.
	We are introducing these fields early to make adding new features easier in the future. Until the features become available and officially supported, we recommend explicitly defining the `panels` in your snippets with only your relevant model fields.
	```