feat(blog): add blog layout example site

[devin-ai-integration]

Summary

Adds a blog/ example site so the blog changelog layout has a Fern-owned demo to point at — docs screenshots and videos currently have to borrow a customer site.

The example is deliberately a full exercise of the layout's surface rather than a minimal one, since it doubles as the reference authors copy:

# blog/fern/docs.yml
navigation:
  - blog: ./blog          # alias of `changelog:`, defaults the title/slug to Blog//blog

# blog/fern/blog/overview.mdx
layout: blog
blog-layout: list
authors:                  # registry the posts reference by ID
  mira: { name, role, url }

Across the six entries: single author, multi-authors, per-post slug (full site-relative, blog/<name>), tags, a hotlinked thumbnail image, and an excerpt override.

Two things the example deliberately does not demonstrate, because they don't work end-to-end yet:

• draft: true / hidden: true only drop the card from the listing. The entry stays in the sidebar nav, stays in /blog.rss, and its page still returns 200 — verified on this PR's preview.
• Local authors.*.avatar paths never resolve: the CLI rewrites and uploads local paths for the image frontmatter key and markdown body images, not nested author fields, so the avatar is dropped and the byline renders name + role only. External avatar URLs do work.

thumbnail vs image, found while building this: the docs describe thumbnail and image as interchangeable for setting the card/hero picture. On the deployed preview they aren't — image only reaches og:image metadata (social-share preview), it never renders on the page in either grid or list layout. thumbnail is the field that actually shows the picture, and only blog-layout: list displays it (beside the post text); grid cards render with no image slot at all. All six posts use thumbnail with a hotlinked URL (no checked-in image assets) and the overview is set to blog-layout: list accordingly. Worth a docs fix independent of this PR.

Pinned to CLI 5.106.1 in fern.config.json — earlier versions reject the blog: key. fern check passes.

Wired into the check, preview-docs, and publish-docs matrices and the README table, so it publishes to blog.docs.buildwithfern.com on merge like the other examples.

Link to Devin session: https://app.devin.ai/sessions/da27e5183b724ee0953e0cd3f4cbcd06
Open in Devin Desktop: https://app.devin.ai/desktop/session/da27e5183b724ee0953e0cd3f4cbcd06?variant=devin

[devin-ai-integration]

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[github-actions]

🌿 Preview blog: https://fern-docs-examples-preview-pr16-blog.docs.buildwithfern.com

Here are the markdown pages you've updated:

• blog/winter-watering-schedules
• blog/seed-catalog-search
• blog/greenhouse-sensor-streams
• blog/soil-composition-endpoint
• blog/pruning-webhooks
• blog/shipping-zones-frost-dates
• blog
• get-started/overview

[devin-ai-integration]

Context on the last three commits, since the images are being chased from the wrong end.

The images aren't missing because of the layout, the image host, or the frontmatter key. This preview is still serving a docs bundle built before fern-api/fern-platform#14268 merged, and that bundle reads frontmatter image as a plain string:

image: readString(frontmatter.thumbnail) ?? readString(frontmatter.image),

The CLI, however, rewrites image into an object for every value it sees — a local path becomes {type: "fileId", value} and an absolute URL becomes {type: "url", value} (visitFrontmatterImages in packages/cli/docs-markdown-utils/src/parseImagePaths.ts, applied to ["image", "og:image", "og:logo", "twitter:image"]). So the old bundle drops the value regardless of whether it points at a checked-in JPEG or at Unsplash, which is why hotlinking changed nothing and why the payload still renders "image":"$undefined".

Two of the three commits therefore can't have the intended effect:

• blog-layout: grid → list doesn't touch image rendering. Both variants render the same BlogCard with the same <FernImage width={1200} height={675}> and the same aspect-video w-full rule; they differ only in flex direction and image width.
• Hotlinking is a no-op for this bug, and it costs the example the local-path demonstration — which is exactly the authoring shape #14268 fixed and the one a reader is most likely to copy.

thumbnail is the one experiment that will appear to work, but for a reason that doesn't generalize: thumbnail isn't in the CLI's rewrite list, so it stays a plain string and the old bundle's readString picks it up. That also means a local thumbnail path is never uploaded, so it only works for absolute URLs.

Suggestion: revert all three and let the merged fix deploy. image: ./images/*.jpg with the checked-in JPEGs and blog-layout: grid is both the better example and the configuration that exercises the fix.