[Feature] Broken Link Checker

[dannysheridan]

Problem description

Identify broken links in Fern Docs. Throw warnings in the Fern CLI.

Why would it be useful?

SEO boost and improved navigation experience.

Additional context

It should identify that this header on the page doesn't exist and warn. (We have moved #audiences to it's own page.)

You can use [audiences in your OpenAPI Specification](/learn/api-definition/openapi/extensions#audiences), too. 

[hkosova]

Link checker wishlist:

• check both relative links ../../path/to/etc and root-relative links /path/to/page
• check anchors: [...](#anchor) (within same page) and [...](anotherpage#anchor)
  • be aware of custom section anchors ## Heading [#custom-anchor]
• check image src - at least for ![](...) and <img src="..."> syntaxes, also in <Frame> components
• check links to snippets <Markdown src="../path/to/snippet.mdx" />
• check card links <Card href="...">
• check links inside snippets:
  • check root-relative links /path/to/page, image srcs, etc.
  • relative links (../path/to/smth) in snippets are tricky because it depends on how they are resolved - relative to the snippet location or relative to the page where the snippet is used. Might be OK to not check relative links and instead advise the users to use root-relative links /path/to/page in snippets because they will be more stable.
• check canonical-url in frontmatter
• check redirect targets (redirects[].destination) in docs.yml, except bulk (:slug) redirects

Also if possible:

• check links to the API endpoint pages
• check #anchor links to individual parameters/fields/sections on API endpoint pages

Nice to have:

• if the tool will be able to check external links, have an option to ignore external links
• option to set the error level for broken links (error vs warning)