docs: add documentation for draft sidebar behavior

[HiDeoo]

Description

• Closes Warning about build issue for draft pages #2783

As discussed today during the Talking & Doc'ing session in Discord, this PR adds a new section in the draft frontmatter field documentation to document sidebar behavior for drafts.

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Tracked Files

Locale	File	Note
en	reference/frontmatter.md	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[delucis]

Small suggestion for your consideration:

• Avoids the “sidebar links […] in your sidebar config” repetition
• Tries specifying “directories” for the second option here to clarify the relationship between page and group

Suggested change

	Because draft pages are not included in build output, you cannot add sidebar links to draft pages in your site sidebar config using [slugs](/guides/sidebar/#internal-links).
	Links to draft pages are not included in [autogenerated sidebar groups](/guides/sidebar/#autogenerated-groups).
	Because draft pages are not included in build output, you cannot add draft pages directly to your site sidebar config using [slugs](/guides/sidebar/#internal-links).
	Draft pages in directories used for [autogenerated sidebar groups](/guides/sidebar/#autogenerated-groups) are automatically excluded in production builds.

Happy for you to decide what to do with this! In general the change is definitely and improvement — thanks @HiDeoo 💜