[docs] WIP: Fix HTML validation issues from link-check

[Janpot]

WIP — enables html-validate's mui:recommended ruleset in the link-check script and applies shared fixes that knock the issue count from 1,342 down to 187:

• Table.tsx: TableCellInner span → div (markdown <p> no longer ends up under <span>).
• CodeBlock.tsx: default the context's codeId/titleId to undefined so Content no longer emits id="" outside a CodeBlock.Root.
• ReferenceTable SectionHeading and AdditionalTypes: h4 → h3 to keep heading levels contiguous under the page's h2.
• SearchBar: existing local change kept.

Remaining 187 issues are page-/component-specific (DemoPlayground aria-label, summary cells containing CodeBlock divs, a few missing labels and heading-level skips on individual pages, merge-props duplicate id) — out of scope for this PR.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4734
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4734
  

commit: 509c759

[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: 1,246.55 ms ▼-102.32 ms^(-7.6%) | Renders: 53 ⁽⁺⁰⁾ | Paint: 1,936.35 ms ▼-139.38 ms^(-6.7%)

Test	Duration	Renders
Select open (500 options)	52.96 ms 🔺+2.12 ms(+4.2%)	14 (+0)
Menu mount (300 instances)	130.64 ms ▼-18.15 ms(-12.2%)	2 (+0)
Slider mount (300 instances)	142.03 ms ▼-16.43 ms(-10.4%)	3 (+0)
Tabs mount (200 instances)	230.65 ms ▼-15.80 ms(-6.4%)	4 (+0)
Select mount (200 instances)	144.29 ms ▼-10.71 ms(-6.9%)	3 (+0)

…and 7 more — details

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