docs content updates

[skarim]

Docs site updates

FAQ corrections (faq.md)

• Rewrote merging answers to reflect atomic landing behavior (PRs land together, not sequentially)
• Corrected "What happens when you merge a PR in the middle of the stack?" — PRs below always merge with it
• Updated "How does merging differ from a regular PR?" to remove sequential-merge language
• Clarified partial-failure semantics — PRs below the failure point stay merged
• Updated "How does rebase merge work?" to describe atomic operation
• Corrected merge queue grouping bullet — stacks are kept in the same group on a best-effort basis (up to 50% overage), not split by default
• Updated "How can I modify my stack?" to lead with gh stack modify and link to the restructuring guide, keeping unstack/init --adopt as an alternative

Authentication callouts (quick-start.md, reference/cli.md)

• Added callout in both the Quick Start and CLI Reference installation sections noting that OAuth auth via gh auth login is required — PATs are not supported

Rebase Stack button documentation (guides/ui.md, faq.md, introduction/overview.md)

• Added new "Rebasing from the UI" section to the UI guide describing full behavior: rebases on latest trunk, cascades through every unmerged branch, force-pushes results
• Added callout that server-side rebase commits are not signed
• Updated FAQ and Overview rebase references with more detail and links to the new UI guide section

[Copilot]

Pull request overview

Documentation updates to align the gh-stack doc site with current Stacked PR behavior in GitHub (atomic stack landing, UI-driven rebases), and to clarify authentication requirements for using the gh stack CLI.

Changes:

• Update FAQ merge semantics to describe atomic landing behavior, partial-failure behavior, and merge-queue grouping details.
• Add/expand documentation for the Rebase Stack UI button (including force-push behavior and unsigned server-side rebase commits) and link to the new UI guide section.
• Add an authentication callout in Quick Start and CLI reference stating OAuth auth via gh auth login is required (PATs not supported).

Show a summary per file

File	Description
docs/src/content/docs/reference/cli.md	Adds an installation callout describing required authentication method.
docs/src/content/docs/introduction/overview.md	Expands the overview’s UI rebase description and links to the detailed UI guide section.
docs/src/content/docs/guides/ui.md	Documents “Rebasing from the UI”, including cascading behavior, force-push, and commit-signing caveat.
docs/src/content/docs/getting-started/quick-start.md	Adds an authentication callout to the Quick Start flow.
docs/src/content/docs/faq.md	Rewrites multiple FAQ answers to reflect atomic landing behavior, UI rebase behavior, merge-queue grouping, and stack modification guidance.

Copilot's findings

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• Files reviewed: 5/5 changed files
• Comments generated: 2

[AdamShwert]

Just one question, but things seem good to me!