docs: deprecate delete_head_branch action

[JulianMaurin]

The delete_head_branch action is deprecated (engine#27721) and will be
removed on June 30, 2026. Users should use GitHub's built-in auto-delete
branches setting instead.

• Add deprecation banner to the action page
• Update frontmatter title to match post_check pattern
• Remove the delete-head-branches use-case page and nav entry

Refs: MRGFY-6799

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

[mergify]

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[Copilot]

Pull request overview

Updates the documentation to reflect the deprecation of the delete_head_branch workflow action and guides users toward GitHub’s built-in auto-delete branches feature ahead of the June 30, 2026 removal.

Changes:

• Add a deprecation banner and update frontmatter (title/description) on the delete_head_branch action doc.
• Remove the “Delete Head Branches” use-case doc page.
• Remove the corresponding “Delete Head Branches” entry from the Workflow Automation → Use Cases navigation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/content/navItems.tsx	Removes the “Delete Head Branches” use-case nav entry to avoid pointing to a removed page.
src/content/docs/workflow/delete-head-branches.mdx	Deletes the dedicated use-case page now that the feature is deprecated.
src/content/docs/workflow/actions/delete_head_branch.mdx	Marks the action as deprecated via frontmatter updates and a prominent deprecation banner with guidance/link to GitHub settings.

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Pull request has been modified.

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-04-02 09:33 UTC · Rule: default
• ✅ Checks passed · on draft merge queue: embarking main (ee4e34d) and #10791 together #10812
• ✅ Merged — 2026-04-02 09:36 UTC · at fc661fd064cbac99a537c01ca8c42dc8f7bf3019

This pull request spent 2 minutes 24 seconds in the queue, including 2 minutes 10 seconds running CI.

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