Fix the docs "View & Edit" link

[julien-deramond]

The "View & Edit" link on every v6 docs page was a 404. Astro's glob() loader gives docs entries an id without the file extension, so the link pointed at .../site/src/content/docs/getting-started/install instead of .../getting-started/install.mdx. The link now adds .mdx and points at main instead of the v6.0.0-alpha.1 tag, since GitHub won't let anyone edit a file from a tag.

Closes #43060.

What was done

• One line in site/src/components/PageMeta.astro: blob/v${current_version}/site/src/content/docs/${id} became blob/main/site/src/content/docs/${id}.mdx.
• All 141 docs pages are .mdx (no .md left under site/src/content/docs), so a fixed extension is enough. Checked blob/main/site/src/content/docs/components/button (404) against .../button.mdx (200).
• Left the other blob/v[[config:current_version]] links (contribute and approach pages) and the Example/Code shortcodes alone: they are source links tied to the released version, and the issue doesn't cover them.
• Kept blob rather than GitHub's edit/main/... URL, as v5 does, because the label is "View & Edit".
• Not run: the docs build and linting. The change is a string in a template.

How to review

• Try it: npm run docs-dev, open any docs page (for example Getting started → Install), click "View & Edit", and check that it opens https://github.com/twbs/bootstrap/blob/main/site/src/content/docs/getting-started/install.mdx with GitHub's edit (pencil) button.

🤖 Generated with Claude Code

[julien-deramond]

@mdo FYI I chose for now to not apply this change manually to gh-pages as I suppose it's not that a problem right now. But if you think it would be better, especially because it's an alpha to help folks contributing, feel free to ping me here, and I'll do the changes.