docs updates

[skarim]

• Added a custom responsive navigation header to the documentation site via the new CustomHeader.astro component and supporting CSS, improving navigation on both desktop and mobile.
• Improved and clarified documentation copy, including FAQ, overview, and guides, and updated screenshots for accuracy.

[Copilot]

Pull request overview

Updates the documentation site experience and copy, including a new responsive header navigation, plus small CLI feedback-link plumbing changes.

Changes:

• Add a custom responsive header/nav component to Starlight (desktop links + mobile hamburger) and supporting CSS.
• Update docs copy/examples (including introducing gs alias usage) and refresh UI screenshots.
• Switch feedback links from a GitHub Discussions URL to gh.io short links and update related tests.

Show a summary per file

File	Description
skills/gh-stack/SKILL.md	Updates CLI usage snippets to show flags before positional args.
README.md	Aligns command synopsis formatting and updates gh stack alias usage ordering.
docs/src/styles/custom.css	Adds responsive show/hide rules for the custom header links vs hamburger menu.
docs/src/content/docs/introduction/overview.md	Adjusts headings/copy and adds gh stack checkout mention.
docs/src/content/docs/index.mdx	Updates getting-started examples (adds alias step, uses gs).
docs/src/content/docs/guides/workflows.md	Updates workflow examples to use gs and clarifies step wording.
docs/src/content/docs/guides/ui.md	Swaps screenshot reference to the new merge box image.
docs/src/content/docs/faq.md	Minor copy improvement around opening PRs as a stack.
docs/src/content.config.ts	Extends docs schema to add a default “private preview” banner.
docs/src/components/CustomHeader.astro	New custom header component (nav links + hamburger dropdown + script).
docs/src/assets/screenshots/stack-merge-box.png	Adds updated screenshot used in docs.
docs/astro.config.mjs	Wires the custom header by overriding Starlight’s SocialIcons component.
cmd/submit.go	Uses the new feedbackURL in generated PR body footer.
cmd/submit_test.go	Updates test expectations to the new feedback URL constant.
cmd/feedback.go	Replaces the old discussions URL with feedbackURL / feedbackFormURL and updates browsing behavior.

Copilot's findings

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Comments suppressed due to low confidence (1)

cmd/feedback.go:43

• The success message always says "Opening feedback form in your browser..." even when args is empty and the command opens feedbackURL (which may be a non-form landing page). Consider making the message match the actual target (or vary it based on whether a title was provided).

	if err := b.Browse(targetURL); err != nil {
		return err
	}

	cfg.Successf("Opening feedback form in your browser...")

• Files reviewed: 14/16 changed files
• Comments generated: 3