[docs] Design changes here and there

[aarongarciah]

Preview: https://deploy-preview-4875--base-ui.netlify.app/react/overview/quick-start

• Makes header, left sidebar, and right sidebar fixed.
• Makes the docs sidebars and main content columns match the home page layout.
• Removes grid lines and footer.
• Removes body's gray background.
• Changes the "Show code" button on demos to be a floating pill.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4875
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4875
  

commit: 334dbde

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	0B(0.00%)	0B(0.00%)

Details of bundle changes

Performance

Total duration: 1,099.16 ms -50.29 ms^(-4.4%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 1,663.13 ms -78.66 ms^(-4.5%)

Test	Duration	Renders
Checkbox mount (500 instances)	57.78 ms ▼-24.02 ms(-29.4%)	1 (+0)

11 tests within noise — details

---

Check out the code infra dashboard for more information about this PR.

[netlify]

✅ Deploy Preview for base-ui ready!

Name	Link
🔨 Latest commit	4a8a91d
🔍 Latest deploy log	https://app.netlify.com/projects/base-ui/deploys/6a19685b42f19100088a75bb
😎 Deploy Preview	https://deploy-preview-4875--base-ui.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
🤖 Make changes	Run an agent on this branch

---

To edit notification comments on pull requests, go to your Netlify project configuration.