feat: Links in markdown when serving .md files

[icyJoseph]

This could be seen a as nice to have, but when serving markdown, the links in the content should also have .md in the pathname.

For example https://docs.stripe.com/testing.md lists some .md links. In react.dev docs markdown pages, this would looke like, /learn/your-first-component.md

This syntax is called [JSX](/learn/writing-markup-with-jsx.md), and it lets you embed markup inside JavaScript.

I guess, we should make those absolute too... this PR doesnt' do that currently. We'd just need to tweak the NOOP_ORIGIN logic.

I am also adding this check:

   if (!fullPath.startsWith(CONTENT_ROOT + path.sep)) {
      continue;
    }

Without, locally in dev or build+start, I can make a request to: curl "http://localhost:3000/api/md/..%2F..%2FCONTRIBUTING" and get the root CONTRIBUTING.md document - only .md files are affected, and the deployed version is protected by Vercel.

[meta-cla]

Hi @icyJoseph!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!

[meta-cla]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[github-actions]

Size changes

Details

📦 Next.js Bundle Analysis for react-dev

This analysis was generated by the Next.js Bundle Analysis action. 🤖

This PR introduced no changes to the JavaScript bundle! 🙌