feat: add redirects for DSG pages

[orinokai]

Summary

Ensure that SSR and DSG pages are redirected to the appropriate handler function. Previously, only SSR pages were redirected and DSG pages were picked up by a catch-all redirect in netlify/netlify-plugin-gatsby that was also inadvertently handling 404 pages.

Test plan

1. Build and deploy the demo in netlify/netlify-plugin-gatsby, ensuring that DSG is enabled in the post.js template
2. During the build, before the netlify.toml is reverted, ensure that /blog/[slug] routes are individually redirected to the handler function

Relevant links (GitHub issues, Notion docs, etc.) or a picture of cute animal

See also netlify/netlify-plugin-gatsby#334

Standard checks:

• Check the Deploy Preview's Demo site for your PR's functionality
• Add docs when necessary

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🧪 Once merged, make sure to update the version if needed and that it was published correctly.

[ericapisani]

LGTM 🚀

[ascorbic]

Looking good!