Documentation: Document page attributes components and a way to have drafts as parent posts

[jorgefilipecosta]

Fixes: #9089

In the classic editor, it has possible to change the parent attributes UI to allow drafts to be selected as a parent of a page/post.
@youknowriad suggested a reasonable way to do that in the new editor would be to implement Page Attributes as a sidebar and remove the existing panel.

This PR documents the set of page attributes components that help having a page attributes sidebar (they were undocumented).
It changes the PageAttributesParent component to allow it to receive the possible parent items as a prop if not passed it queries the items as before. This simple change allows plugin developers to use all the logic of the component but just pass a different prop with the items they want e.g.: draft posts and the subset resulting of some specific filter.
And in the end, we provide a code sample that does precisely what @youknowriad suggested implements page attributes in a sidebar, with the possibility of allowing draft posts to be selected as a parent and removes the existing Page attributes panel.

How has this been tested?

I pasted the code sample in the browser console, and I checked it works correctly.

[chrisvanpatten]

I don't want to let perfect be the enemy of good so I'm not approving/denying, but there are some small docs things it'd be nice to fix. Cool to see this though!

[chrisvanpatten]

I think generally if these lines are intended to be separate paragraphs there should be a newline between each line. Otherwise, they can all be one line (we don't do line-wrapping in markdown).

[chrisvanpatten]

I believe these should be H2s — we should only have a single H1 per page.

[chrisvanpatten]

We should also standardise on a single type of heading syntax: # or =. I prefer # but this isn't a blocker, just an observation. (I might look for markdown linters today and write a proposal for including one.)

[chrisvanpatten]

This should have an H1 page title :)

[jorgefilipecosta]

Thank you for your review @chrisvanpatten all your feedback was applied!

[oandregal]

Documentation seems fine. Tested that the example works as expected. This would be the result (no page attributes component will be shown in the document sidebar and a new sidebar will be added for this):

I'm a bit confused by this behavior, though. I'd think that we either want to:

1. fix the current component and allow for drafts (if it is a backward compatibility issue).
2. allow plugins to customize the parent checkbox contents (if what we want is to allow extensibility).

I'd want a second opinion on this before approving @youknowriad @jasmussen @mapk

[jorgefilipecosta]

Hi @nosolosw thank you for your review.
In issue #9089 it is possible to see the discussion that happened before arriving at this solution.
The base idea is that we don't want a very granular filter to allow this extensibility mechanism while we already have some possible alternatives.
This PR documents the solution referred by @youknowriad in #9089 (comment):

Developers can remove the panel and register a custom sidebar, This is a workaround.

[youknowriad]

Is this still needed? Maybe not with the new Document Slot?

[jorgefilipecosta]

Hi @youknowriad, the fact that "In the classic editor, it has possible to change the parent attributes UI to allow drafts to be selected as a parent of a page/post." and in Gutenberg, we don't have an easy way to do that still persists.
Maybe this PR should be updated to document how to do this using the Document Slot?

[youknowriad]

Maybe this PR should be updated to document how to do this using the Document Slot?

Works for me 👍

[cr0ybot]

After a recent change, PageAttributesParent no longer accepts props, which breaks the proposed code... seems like either replace the dropdown entirely or provide a filter for the query.

[tomdevisser]

@jorgefilipecosta @youknowriad Is this still relevant? What needs to be done in order for this PR to be moved forward/closed? :)

[jorgefilipecosta]

I think we can close this PR.