Docs: add a <main> landmark to the homepage and the 404 page

[julien-deramond]

The homepage and the 404 page have no <main> landmark. axe's landmark-one-main rule fails on both, and the "Skip to main content" link lands on a plain div on the homepage and on nothing at all on the 404 page.

Every other page gets its <main> from its layout through the main slot. @twbs/bui's BaseLayout is supposed to wrap pages that don't fill that slot in <main id="content">, but our BaseLayout always fills it, so that fallback never runs and the page content is rendered bare. Now our BaseLayout renders <main id="content"> itself in that case, with the page's overrides.main attributes. The masthead drops its own id="content", and the 404 page drops the id: 'content' it passed, since the layout sets it.

Closes #43045.

The homepage doesn't change: the screenshots before and after are identical at 1280px and 390px. The 404 page does, because its overrides.main (my-auto p-5) was never applied while no <main> was rendered. With d-flex flex-column min-vh-100 on the body, the message is now centered vertically and the footer sits at the bottom, as the page's markup intended.

Before	After

I fixed the layout instead of turning the masthead into <main> (as #43102 does), because that only fixes the homepage and leaves the 404 page without a landmark. In the fallback, id: 'content' comes before the spread of mainProps, so a page that passes its own id replaces it instead of printing a second id attribute.

In the built site, every page rendered by BaseLayout now has exactly one <main>. The few pages still without one are the standalone snippet examples, which have their own markup. I didn't run axe, Lighthouse, or a screen reader.

Live previews

• https://deploy-preview-43105--twbs-bootstrap.netlify.app/
• https://deploy-preview-43105--twbs-bootstrap.netlify.app/404.html

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