docs site content updates

[skarim]

• Added private preview callout banners
• Expanded squash merge FAQ with examples
• Clarified bottom-up merge process
• Clarified CLI as optional with guidance on how to use with other tools

[Copilot]

Pull request overview

Updates the Stacked PRs docs site to better set expectations for private preview availability, clarify merge behavior for stacks, and expand documentation around squash-merge rebasing and CLI-optional workflows.

Changes:

• Added “Private Preview” callouts (home + Quick Start) to prevent confusion when the feature isn’t enabled.
• Clarified bottom-up / contiguous-group merge behavior and expanded guidance that the gh stack CLI is optional (including “bring your own tools”).
• Expanded the squash-merge FAQ with a concrete git rebase --onto explanation and example.

Show a summary per file

File	Description
docs/src/styles/custom.css	Styles inline code in the hero tagline and improves copy-button visibility for light-themed code blocks.
docs/src/content/docs/introduction/overview.md	Adds a squash-merge handling callout and expands CLI-optional + BYO tools guidance with FAQ links.
docs/src/content/docs/index.mdx	Updates homepage messaging to emphasize CLI optionality and adds a Private Preview caution aside.
docs/src/content/docs/guides/stacked-prs.md	Clarifies bottom-up merging to explicitly allow contiguous-group merges from the lowest unmerged PR.
docs/src/content/docs/getting-started/quick-start.md	Adds a Private Preview caution banner near prerequisites.
docs/src/content/docs/faq.md	Significantly expands the squash-merge explanation with rebase --onto and an illustrative example.

Copilot's findings

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• Files reviewed: 6/6 changed files
• Comments generated: 0

[AdamShwert]

Didn't see any typos, content seems good!