[PageLayout] Add new layout option PageLayout.Sidebar

[TylerJDev]

Closes #

PageLayout handles the layout for many pages across GH, and comes with 4 sub-components to define a layout (header, content, pane, footer). This PR adds an additional sub-component for side content.

We currently utilize pane to encapsulate content adjacent to the content and directly underneath the header. We do not offer a way to "escape" the main layout and add content adjacent to it.

Changelog

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• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
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Testing & Reviewing

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

[changeset-bot]

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Latest commit: 6113e5b

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Or, apply the integration-tests: skipped manually label to skip these checks.

[primer-integration]

👋 Hi from github/github-ui! Your integration PR is being built, you can follow the progress here: https://github.com/github/github-ui/actions/runs/21870400950

[primer-integration]

Integration test results from github/github-ui:

  CI Passed
  VRT Passed
  Projects Passed

All checks passed!