[docs-infra] Move focus to the main content with the skip link

[lazerg]

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Fixes #48943

Activating "Skip to content" scrolled the page but left DOM focus where it was, so the next Tab went back through the announcement bar and the header. MarkdownLinks registers a document-level click handler that calls preventDefault() on every internal anchor and hands the URL to Router.push, and the skip link goes through it as well, so the browser never runs its own fragment navigation.

Opting the link out with data-no-markdown-link brings that back, and #main-content now carries tabIndex={-1} so it can receive focus.

Validation:

• pnpm test:e2e-website:dev -g "skip link", which fails on master and passes here
• pnpm eslint, pnpm typescript and pnpm prettier --check on the touched files

[code-infra-dashboard]

Deploy preview

https://deploy-preview-48944--material-ui.netlify.app/

Bundle size

Bundle	Parsed size	Gzip size
@mui/material	0B(0.00%)	0B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

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