associate BlockMarkup.md with the BlockMarkup type

[tayloraswift]

this markdown article is running into a DocC bug where it replaces the BlockMarkup symbol, which causes some rather strange rendering:

this PR associates the article properly to the BlockMarkup symbol by turning the article into a markdown supplement.

[d-ronnqvist]

Since articles are phased as a list of all the nodes I wonder if it's be more according to the original intent to just rename those files to "block-markup-nodes.md" and "inline-markup-nodes.md".

Also, @DocumentationExtension(mergeBehavior: append) is the default and is never needed. Please remove that.

[tayloraswift]

i don’t think these these stubs would be very useful as articles on their own, so i’m assuming the intent was just to add some custom curation to these types.

i’ve removed the redundant block directives.

[d-ronnqvist]

I think the intent was for these pages to be "API collections" just like the "Infrastructure", "Visiting Markup", and "Formatter and Options" pages. An API collection is authored in a markdown file, just like an article, but its purpose is to organize related symbols rather than to describe how to do something. You can find more examples of API collections in Apple's documentation, for example here in UIKit and here in SwiftUI.

[tayloraswift]

cc @Kyle-Ye

[Kyle-Ye]

I think the intent was for these pages to be "API collections" just like the "Infrastructure", "Visiting Markup", and "Formatter and Options" pages. An API collection is authored in a markdown file, just like an article, but its purpose is to organize related symbols rather than to describe how to do something. You can find more examples of API collections in Apple's documentation, for example here in UIKit and here in SwiftUI.

Here is what I got from @tayloraswift 'description.

The name of our article page is the same as some protocol in Markdown framework which will cause some strange rending issue when conforming the protocol elsewhere.

[Kyle-Ye]

The initial intend for these file is to serve as an article. We reference them in Markdown.md with the first one instead of the second.

- <doc:BlockMarkup>
- ``BlockMarkup``

So I'd like to suggest you update the filename of article and update those articles reference in Markdown.md

eg. InlineMarkup.md -> Markup Inline Nodes.md and <doc:Markup-Inline-Nodes>